Carbon nanotube photothermionics: Toward laser-pointer-driven cathodes for simple free-electron devices and systems
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oduction Free-electron devices are ubiquitous. They enable high-power, high-speed, radiation-resistant electronics; x-ray imaging; high-resolution microscopy and patterning; various types of spectroscopy; and materials processing. Electron accelerators have numerous scientific and technological applications, and free-electron lasers provide unique characterization capabilities. Perhaps even more intriguing is the prospect of vacuum micro-/nanoelectronic devices, which may combine highdensity integration and scalable manufacturability with the high-speed and low-loss nature of electron travel in vacuum, or that of tabletop accelerators and free-electron lasers. In addition to its fundamental scientific appeal, in this broad context of applications, rooted in a century-old history and extending far into the future, research on electron emitters represents an exciting quest. The electron affinity or work function of common materials are several electron volts—hundreds of times larger than the average thermal energy of electrons at room temperature. To release electrons into a vacuum thus requires an external excitation. This can be accomplished by applying an electric field, light, heat, or through electron/ion bombardment— or combinations of some of these1—and may be mediated by quasi-particles such as phonons and plasmons. Photocathodes enable control of the emission spot size, shape, and position,
through control of the optical excitation beam. Given the maturity of laser/optics technologies, this is highly appealing. While metallic photocathodes typically have low quantum efficiencies, semiconducting photocathodes with quantum efficiencies greater than 50% at reasonably low photon energies have been reported.2 Nonetheless, sensitive surfaces and associated ultrahigh vacuum requirements often limit applicability. Light-induced heating of a material for thermionic emission (or, more accurately, thermal electron emission) thus represents an appealing alternative. When a beam of light strikes a conductive surface, the heat produced spreads to a far wider area than the illuminated spot; temperature rises throughout the cathode. In order to compensate for this heat loss and achieve thermionic emission temperatures, optical intensities as high as hundreds of kW/cm2 may be necessary, warranting a complex optical system. Further, to prevent the detrimental spread of heat throughout the device, strong thermal insulation of the cathode is required, leading to additional complexity and limiting miniaturizability. More fundamentally, the spread of heat results in an electron emission spot much larger than the illumination spot, where information about the shape of the optical beam is lost, undermining its control over the ensuing electron beam. Recent research has shown that carbon nanotubes (CNTs) can address these challenges
Alireza Nojeh, Department of Electrical and Computer Engineering, Quantum Matter Institute, The University of British Columbia, Canada; [email protected] doi:10.1557/mrs.2017.139
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