Compared to other form of nanostructures, 1D NT/NW have been used the most extensively in electron source assemblies. (21) Its axial symmetry is compatible with an electron microscope column and its extended side surfaces provide sites for attaching to emitter support needle. (22) However, because of this side attachment configuration, axial symmetry of electric field distribution around the NT/NW tip could vanish. Such effect becomes more pronounced when short suspension length of NT/NW is used to reduce mechanical vibration. As a result, even though the NT/NW body is perfectly centered laterally and aligned angularly, the emitted electron beam will still be out of axis. To solve this problem, we used FIB to reshape the support needle tip into a collimator structure, as shown in the SEM image of figure 1a. The yellow pseudo color represents a collimator structure which is used to form axially symmetric electric field around the emission tip of a nanowire; Blue pseudo color represents a carbon pad that is used to stop chemical reaction between nanowire and metal support body during filament heating; Purple pseudo color represents the LaB6 nanowire position. Height of the carbon pad is adjusted by FIB milling to ensure that the nanowire is positioned at the collimator center. Figure 1a inset is an optical microscope image of a TEM emitter showing an arch-shaped filament and the position of the fabricated microstructure at a Ta needle tip. The SEM snapshots of crucial steps of installing a LaB6 nanowire into this microstructure are presented in figure 1b-d. In the first step, one LaB6 nanowire was attached on a nanomanipulator hand through Van Der Waals force. Such nanowire was then transported through the collimator hole by driving the manipulator hand. (Figure 1b) After the nanowire got into full contact with the carbon pad, electron beam induced carbon deposition was applied to the bonding region to fix the nanowire. (Figure 1c) At the last step, manipulator hand was retracted and detached from the nanowire. It was realized because the Van Der Waals force is much weaker compared to carbon bonding force. (Figure 1d). Figure 1e and f showed a procedure where two manipulator hands are placed onto support needle at positions marked by two solid arrow heads. When the two manipulator hands moved in the direction of arrow heads, microscale bending of the support needle tip was realized. The bending amount was pre-determined by measuring off-axis angle using field emission pattern, as shown in figure 1e inset. (1) In this case, even though the LaB6 nanowire body (marked by a hollow arrow head) was already aligned with support needle axis, emission pattern still showed a 2.5-degree angle out of axis. Figure 1f is an SEM image of the emitter tip after 3-degree micro-bending performed by the two manipulator hands. Inset shows a perfectly aligned field emission spot on the phosphor screen center hole after such a set of manipulations.
In order to validate the above described LaB6 NW emitter in a commercial TEM, a 60kV electron gun and high voltage tank were customized to adapt the changes in extraction voltage and total emission current. The extraction voltage and emission current for the LaB6 nanowire emitter are ~400V and ~40nA, which are much lower than ~4000V and ~15uA, as used by conventional W (310) emitter. A TEM equipped with such a LaB6 NW electron gun is shown in figure 2a. Components marked along beam path from top to bottom are the NW emitter, condenser lens aperture (CLA), specimen, high angle annular dark field (HAADF) detector, Faraday cage for probe current measurement, image screen, and EELS spectrometer. Figure 2b is a defocused TEM image about the CLA, taken after emitter centering with respect to the aperture using a gun motor capable of lateral movement. (23) Transmitted electrons and those scattered by the aperture edge form Fresnel diffraction fringes in the nearfield. The visibility of such fringes on the TEM screen is determined by an amplitude damping envelop formed due to lens aberration. Therefore, axially symmetric fringe pattern is observed only when the electron source is placed on the axis passing through the CLA center and thereby forming the same angles to the entire aperture edge circumference. Only a small portion of the total emitted electrons could go through the apertures and reach the image screen if emitter angular pre-alignment is poor. In this case of perfect pre-alignment, more than 10nA beam current could reach the image screen for low magnification TEM observation with large field of view. Figure 2c is a 30kx low-magnification TEM image showing a LaB6 nanowire as synthesized by chemical-vapor-deposition method. Amplitude contrast is the main TEM image forming mechanism at this magnification. To demonstrate phase-contrast imaging, a Si  specimen was used to take high resolution TEM (HRTEM) images with magnifications from 400kx to 1000kx. A region from a 1000kx HRTEM image is displayed in figure 2d to show clear Si (111) planes. Figure 2e is a fast Fourier transform (FFT) diffractogram of the HRTEM image. (Sfigure1 in supplementary information) The highest observable spatial frequency corresponds to 90pm spacing of Si (442) planes. This spot is likely to be produced by secondary diffraction from two diffracted beams, because (442) is a forbidden reflection in primary diffraction pattern of diamond structure crystals. (24) The second smallest observable feature corresponds to 96pm of Si (440) planes. Two vectors were drawn in the diffractogram connecting the center transmitted spot to one 90pm spot and its neighboring 96pm spot respectively. The 96pm vector passes through Si (220) primary reflection and the 90pm vector passes through no primary reflections. Based on the principle of transmission cross-coefficient, non-linear phase contrast transfer function in the reciprocal space reaches maximum along primary reflection directions. (25, 26) High intensity damping occurs for secondary reflections that are out of those primary reflection directions, especially for high index reflections. If 96pm spot is also a secondary reflection, it would therefore be much brighter than 90pm spot as a result of higher phase contrast transfer. However, the fact that 96pm spot is weaker in intensity compared to 90pm spot suggests that 96pm spot is a primary reflection which intensity is damped by lens aberration envelop. Therefore, an upper bound of information limit is estimated to be 96pm. Information limit of this TEM using the conventional W (310) emitter is 110pm~120pm. (27) The finer resolvable features by the LaB6 NW electron source is a proof of higher spatial temporal coherence, which determines the damping envelop in a linear phase contrast transfer process. (28, 29)
It requires detailed knowledge of imaging condition and simulation to evaluate resolution quantitatively for an HRTEM image with spherical aberration. (30) STEM is a probe-forming mode of TEM. A demagnified image of the electron source forms an electron probe which is used to interact with specimen atom columns in raster scan manner. Image resolution can be directly measured by finding the smallest feature resolved in a STEM image. Figure 3a is a STEM image taken with HAADF detector about the Si  specimen, which is the same as that used for the TEM image of figure 2d. Each white dot in figure 2d is now resolved to be a dumbbell structure with 136pm atomic separation. The inset is a diffractogram of the image. The reflection with the highest observable spatial frequency is Si (440) with 96pm lattice spacing. Four Si atom columns marked by a red rectangle were magnified in figure 3b and their line profile is displayed below the image. The specimen thickness was determined to be 50nm by EELS method. (31) A multi-slice simulation at 60kV was then carried out by QSTEM to fit the image line profile with the assumption that the electron probe shape is a convolution between aberration-related broadening and a Gaussian shaped source image. (32, 33) The best fit was found when source image FWMH was set to be 60pm and it resulted in a probe shape with FWMH of 88pm. (Sfigure2 in supplementary information) Single atom imaging capability was demonstrated by STEM imaging of a single layer graphene specimen, which is presented in figure 3c. Three pairs of graphite (200) reflections with 107pm spacing were clearly shown in figure 3d, which is a diffractogram of figure 3c. Four neighboring C atoms in figure 3c as selected by a red rectangle were used to make a line profile shown in figure 3e. The same electron probe with 88pm FWHM was again used to simulate a graphene image. The well-matched line profile thereby verified our previous probe size estimation.
To take advantage of the high current stability of the LaB6 nanowire emitter in moderate UHV condition, our customized electron gun was only pumped by ion pumps to keep vacuum in the ~10-8Pa range, while the original W (310) electron gun vacuum was in the ~10-10Pa range enhanced by non-evaporable getter pumps. (34) As figure 4a shows, 21% of total emission current from a LaB6 nanowire emitter could be used as probe current, which is measured using a Faraday cage near the image screen. Low noise level in the probe current is especially important for single atom HAADF imaging of low atomic weight elements because of their small electron scattering cross-section. In the case of graphene, signal-to-noise ratio is typically ~10%. (35) To allow 8-bit greyscale information, the noise level of STEM probe current is expected to be below 0.4%. Figure 4b is the probe current profile measured for 1 minute, which is longer than the usual STEM frame acquisition time. A peak-to-peak noise ratio of 0.36% was demonstrated, even after a full day of use without emitter heat treatment. EELS is an indispensable spectroscopy method that has become standard to combine with STEM to realize atomic resolution chemical analysis. (36) While its spatial resolution depends on STEM probe size, its energy resolution depends on probe temporal coherence, which can be measured as the FWHM of ZLP. Electron probe energy spread originated from emitter energy spread and column related factors such as instability of high voltage tank, objective lens and EELS spectrometer, all contribute to the final measured ZLP width. To compare the ZLP between LaB6 NW electron gun and W (310) electron gun on equal ground, we acquired ZLPs from both electron guns using the same column, spectrometer and high voltage tanks with negligible noise compared to the expected energy spread range. Figure 4c includes ZLPs from LaB6 NW emitter, W (310) emitter and Schottky emitter all acquired at 60kV acceleration voltage and ~15pA probe current. 0.22eV, 0.28eV and 0.8eV were measured as FWHMs for the three electron guns respectively. ZLP FWHMs with respect to probe current were also plotted for both the LaB6 NW electron gun and W (310) electron gun in figure 4d. At probe current ~5pA, both electron guns reached the narrowest ZLPs which are 0.2eV and 0.27eV respectively. (Sfigure3 in supplementary information) It is noted that the LaB6 nanowire electron gun energy spread is consistent with our previous measurement using hemispherical analyzer in a low voltage field emission chamber. (1) However, energy spread from W (310) emitter was much higher in our previous measurement. It is due to the >100-fold higher vacuum in the JEM-Arm200F TEM electron gun where W (310) could maintain a clean surface for a much longer time. On the other hand, the less vacuum stringency and consistent performance of LaB6 nanowire emitter in a wide range of vacuum conditions make it suitable for applications in various electron microscopes of a wide cost range. (37, 38)
As a conclusion, micro-fabrication and nano-manipulation were carried out to make perfectly angularly aligned LaB6 NW electron source. Such an electron source was installed in an aberration-corrected (S)TEM where sub-angstrom features in both broad-beam mode and probe-forming mode were resolved at 60kV acceleration voltage. The higher spatial temporal coherence offered by the nanostructured electron source produced advantages over the state-of-the art W (310) electron source in both microscopic spatial resolution and spectroscopic energy resolution. With the additional merits of low vacuum stringency and probe-forming efficiency from the LaB6 NW electron source, a natural next step would be applying such emitter in high voltage monochromated (S)TEM applications to benefit frontier studies such as atomic resolution vibrational spectroscopy. (39, 40) Another development direction would be a new generation low-cost electron beam imaging and chemical analysis instruments with not only high performance but also broader applicability and affordability.