Ultrabright and monochromatic nanowire electron sources
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Introduction Electric-field-induced emission of electrons is a physical phenomenon that utilizes a strong electric field to lower the surface energy barrier such that electrons can tunnel into a vacuum without the assistance of thermal excitation. Many types of electron optical instruments require a point electron source that can produce an electron probe with high brightness and monochromaticity for imaging and analysis of samples under investigation. Over the past two decades, research on nanostructures of materials has resulted in numerous innovations of nanoscale electron emission structures including single atom tips, nanotubes, and nanowires. Field emission is well described by the Fowler–Nordheim equation,1 developed in 1928, in which the electron emission current density (in A/m2) is obtained as: J=a
φ3/ 2 ¬ F2 , exp b φ F ®
(1)
where a = 1.54 × 10 −6 A eV V–2, b = 6.83 × 10 9 V m–1 eV–3/2, ϕ is the work function (in eV) of the emission surface, F is the local electric field (in V m–1), and β is a dimensionless V field enhancement factor defined as F = β , where V is the r
applied extraction voltage and r is the radius of curvature of
the sharp tip from which electrons are emitted. The strength of the electric field F depends on the geometrical sharpness of the emitter tip.2 Cold-field emission provides the highest brightness and monochromaticity among various electron sources. This is especially important for electron optical applications. In the past, only refractory metals such as W, Mo, and Ta were considered field-emission materials. This was due to several factors. First, mature needle etching processes were only available for these metals to produce the required tip sharpness. Second, thermal flashing, an established procedure to remove the adsorbates on the emitter by controlled heating, was effective to produce an oxide-free clean surface for these metals. For other materials, including carbon or compounds, there is still no effective procedure available that guarantees formation and subsequent cleaning of a sharp needle thinner than 100 nm.3,4 Since its first application in the scanning transmission electron microscope (STEM) to image single atoms in the 1960s, tungsten continues to be the only field-emission electron source for electron microscopes in industrial applications where it delivers the highest brightness and service life.5 However, even for the most refined electron source made of a single crystal of tungsten with its (310) facet as the emission surface denoted as W(310), its applications are still limited by its poor emission stability (5–10%) and large energy spread (0.3–0.4 eV).
Han Zhang, Advanced Low-Dimensional Materials Laboratory, National Institute for Materials Science, Japan; [email protected] Jie Tang, Advanced Low-Dimensional Materials Laboratory, National Institute for Materials Science, Japan; [email protected] Jinshi Yuan, Advanced Low-Dimensional Materials Laboratory, National Institute for Materials Science, Japan; [email protected] Lu-Chang Qin, Depa
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