Development of Ultra-High Brightness Coherent Carbon Nanotube Electron Sources by Atomic Level Surface Control

为了稳定地增强电子场发射，研究人员追求了分子（或团簇）的强粘附性，并证实了碳纳米管（CNT）发射的电子束具有高相干性，如下所示。不同尖端曲率半径CNTs的场发射场发射显微镜对尖端为锥形、锥角为19.2°的特殊多壁CNTs“d-CNTs（密度最大的CNTs）”进行了场发射显微镜（FEM），得到了两种有限元模式，一种是模糊的斑点模式，另一种是清晰的五边形模式。通过对测量到的尖端曲率半径和场发射阈值电压的检测，得出了斑点图案来源于曲率半径小于约2 nm的尖尖，五边形图案来源于曲率半径大于约2 nm的尖尖。这一结论与之前单壁薄碳纳米管和多壁厚碳纳米管的有限元结果一致。更多地取决于碳纳米管尖端的五边形-五边形距离是否大于或小于碳纳米管中的电子波长（费米波长），这表明碳纳米管中的电子具有较大的相干长度（超过10 nm）。表面有RuO_2的多壁碳纳米管的场致发射通过滴入RuO_4溶液，在多壁碳纳米管上沉积RuO_2簇。当使用适量的RuO_4溶液时，观察到稳定的亮点可能来自直径为1 ~ 2 nm的RuO_2簇，并且阈值电压降低。若使用过量的RuO_4溶液，则电子发射性能变差。将碳纳米管尖端的五角形视为电子波通过的空穴，模拟了远离空穴形成的衍射图样。通过假设发射的电子是一种球形传播波，其源位于空穴后面，模拟的模式很好地再现了实验模式。该模拟支持碳纳米管发射的电子的高相干性。少

Robust adhesion of molecules (or clusters) for stable enhancement of electron field emission was pursued, and high coherency of an electron beam emitted from a carbon nanotube(CNT) was substantiated, as follows.1.Field Emission from CNTs with different radii of tip curvatureField emission microscopy(FEM) carried out for special multiwall CNTs named "d-CNTs(densest CNTs)", which have cone-shape tips with a cone angle of 19.2°, gave two kinds of FEM patterns, i.e., one is vague, speckled patterns and the other clear pentagon patterns. Inspection of the measured radii of tip curvatures and threshold voltages for field emission led to the conclusion that the speckle patterns originate from sharp tips with radius of curvature less than ca. 2 nm and the pentagon patterns originate from tips with radius curvature larger than ca. 2 nm. This conclusion is consistent with the previous FEM results for thin single-wall CNTs and thick multiwall CNTs. The types of FEM patterns from CNTs are supposed … More to be determined by whether the pentagon-pentagon distance on their tip is larger or smaller than the electron wavelength (Fermi wavelength) in CNTs, showing the large coherence length (over 10 nm) of electrons in CNTs.2.Field emission from multiwall CNTs with RuO_2 on their surfacesDeposition of RuO_2 clusters on multiwall CNTs was carried out by dropping RuO_4 solution. When an appropriate amount of RuO_4 solution was used, stable bright spots supposedly originating from RuO_2 clusters (1-2 nm in diameter) and reduction in threshold voltages were observed. When excess amount of RuO_4 solution was used, however, electron emission properties were deteriorated.3.Simulation of FEM patternsPentagons on a CNT tip being regarded as holes where electron wave pass through, diffraction patterns formed away from the holes were simulated. By assuming the emitted electrons as a spherically propagating wave whose source was placed behind the holes, simulated patterns reproduced well the experimental patterns. This simulation supports the high coherency of electrons emitted from a CNT. Less