Electron-emission materials: Advances, applications, and models
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Introduction Electrons represent data carriers for computing and imaging as well as power carriers for electrical energy. As opposed to electron flow in solid-state materials, “free” electrons in a vacuum are unfettered by scattering and recombination mechanisms, providing advantages for electron transfer, manipulation, and imaging. Vacuum electron devices formed the core of early 20th century technology spanning from electronics to thermal-energy conversion, from microscopy to x-ray generation, from imaging to mass spectrometry, from linear accelerators to sensors. Stimulating, enhancing, and controlling emission of electrons and their subsequent flight in space and time has driven the development of low-work-function materials, high-aspect-ratio devices, nanostructures, and radiationabsorbing materials. Electron emission has a rich history that played a pivotal role in vacuum-tube technology and has underpinned the development of modern electronic devices and circuits. At a fundamental level, electron emission relies on excitation of electrons above the material’s work function, typically 3–5 eV, allowing these energetic electrons to exit the material when they encounter the surface. Many different physical stimuli can supply this energy, including thermal-energy (coined as “thermion” emission by Thomas Edison in 1880), photons (the photoelectric effect made famous by Einstein),
ion or electron bombardment, and large electric fields, which, unlike the others, allow electron tunneling toward the vacuum (Figure 1). Understanding these processes led to the invention of the vacuum diode by J.A. Fleming in 1903,1 and the Nobel Prize in Physics to O.W. Richardson in 1928 for the theoretical description and equation for thermionic emission.2 Following on Fleming’s work, L. De Forest developed the vacuum tube “triode,” using a third terminal to provide current control and amplification,3 leading to the subsequent electronic revolution. Devices based on electron emission and emissive materials remain important objectives of 21st century science. Compared to early vacuum-tube technology, today’s applications require higher electron density, narrower electron-energy distribution, shorter emission times, and more efficient excitation. These requirements have propelled the development of new materials and physical emission mechanisms, often taking advantage of the unique electronic and thermal properties of low-dimensionality materials and nanoscale phenomena. The articles in this issue review a number of these new materials and applications.
Recent advances in electron emission The different physical mechanisms inducing electron emission can be classified as photoelectron, secondary electron,
Daniele M. Trucchi, Institute for Structure of Matter, National Research Council of Italy, Italy; [email protected] Nicholas A. Melosh, Department of Materials Science and Engineering, Stanford University, Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, USA; [email protected] doi:10.1
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