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High-endurance micro-engineered LaB6 nanowire electron source for high-resolution electron microscopy

Abstract

The size tunability and chemical versatility of nanostructures enable electron sources of high brightness and temporal coherence, both of which are important characteristics for high-resolution electron microscopy1,2,3. Despite intensive research efforts in the field, so far, only conventional field emitters based on a bulk tungsten (W) needle have been able to yield atomic-resolution images. The absence of viable alternatives is in part caused by insufficient fabrication precision for nanostructured sources, which require an alignment precision of subdegree angular deviation of a nanometre-sized emission area with the macroscopic emitter axis4. To overcome this challenge, in this work we micro-engineered a LaB6 nanowire-based electron source that emitted a highly collimated electron beam with good lateral and angular alignment. We integrated a passive collimator structure into the support needle tip for the LaB6 nanowire emitter. The collimator formed an axially symmetric electric field around the emission tip of the nanowire. Furthermore, by means of micromanipulation, the support needle tip was bent to align the emitted electron beam with the emitter axis. After installation in an aberration-corrected transmission electron microscope, we characterized the performance of the electron source in a vacuum of 10−8 Pa and achieved atomic resolution in both broad-beam and probe-forming modes at 60 kV beam energy. The natural, unmonochromated 0.20 eV electron energy loss spectroscopy resolution, 20% probe-forming efficiency and 0.4% probe current peak-to-peak noise ratio paired with modest vacuum requirements make the LaB6 nanowire-based electron source an attractive alternative to the standard W-based sources for low-cost electron beam instruments.

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Fig. 1: Micro-engineering of a NW electron source.
Fig. 2: Coherent TEM imaging using a LaB6 NW electron gun.
Fig. 3: Aberration-corrected STEM imaging using a LaB6 NW electron gun.
Fig. 4: Unmonochromated EELS energy resolution.

Data availability

Additional data and raw data are available from the corresponding author upon reasonable request.

Code availability

The code that has been used for this work is available from the corresponding author upon reasonable request.

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Acknowledgements

This study was supported by the National Institute for Materials Science Advanced Measurement and Characterization Project.

Author information

Authors and Affiliations

Authors

Contributions

H.Z. carried out the experiments and wrote the paper. Y.J. and A.N. carried out physical evaluation of the LaB6 NW emitter with TEM. A.I. developed high voltage electronics for the LaB6 NW emitter electron gun. A.Y. acquired STEM and TEM data. C.O. and K.K. contributed to the (S)TEM image analysis and simulation. T.K., H.T.M. and H.W. carried out nanomanipulation. N.T. contributed to the NW synthesis. Y.Y., D.F., S.-i.K. and H.M. coordinated the project.

Corresponding author

Correspondence to Han Zhang.

Ethics declarations

Competing interests

The authors Y.J., A.N., A.I., A.Y., S.-i.K. and H.M. are affiliated with JEOL Ltd (Akishima, Tokyo, Japan), which is a producer of electron microscopes, including TEMs.

Additional information

Peer review information Nature Nanotechnology thanks Gregory Schwind and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Collimator effect.

(a) LaB6 nanowire emitter SEM image showing a bare support needle tip without collimator structure; Field emission pattern shows an axial deviation angle of 6o; Field emission electron trajectory calculated for the emitter structure using finite element method; (b) LaB6 nanowire emitter SEM image showing a support needle tip with a collimator structure; Field emission pattern shows an axial deviation angle less than 1o; Field emission electron trajectory calculated for the emitter structure using finite element method; (c) Finite element simulation of the axial deviation angle, ϕ, dependence on nanowire suspension length, l, for the bare needle tip structure (red) and the collimator tip structure (blue). Spade shaped data were experimentally measured.

Extended Data Fig. 2 HRTEM images.

(a) Structure of the LaB6 nanowire TEM. Components labelled along beam path from top to bottom: nanowire emitter, condenser lens aperture, specimen, annular dark field detector, Faraday cage, image screen, and EELS spectrometer; (b) The 800kx HRTEM image taken by using the LaB6 NW emitter. Red rectangle marked the region where the FFT diffractogram of Fig. 2d is created from; (c) That taken by using the original W (310) electron gun with same imaging condition as (b); Red square marked the region used to generate diffractogram through FFT.

Extended Data Fig. 3 Titan3 images.

(a) Energy spread of electron beam produced by a Schottky electron gun with the monochromator in off state; (b) Energy spread of electron beam produced by a monochromated Schottky electron gun. (c) A high-resolution TEM image taken on Si (110) specimen using the beam condition of (a). Inset showing FFT diffractogram; (d) That using beam condition of (b). Inset showing FFT diffractogram.

Extended Data Fig. 4 Titan3 CTF.

(a) Chromatic envelop functions plotted for TEMs using LaB6 nanowire emitter, monochromated Schottky emitter, and unmonochromated Schottky emitter, respectively. Dashed curve representing contrast transfer function for the LaB6 nanowire TEM until the first zero-crossing. (b) Blue and red outlined diffractograms of the TEM images recorded using the Schottky emitter under unmonochromated and monochromated conditions respectively. Three additional reflection spots appearing in the monochromated TEM images, being labelled with their corresponding lattice indices and spacings.

Extended Data Fig. 5 Diffractogram comparison.

(a) Diffractograms from the W (310) TEM (outlined in red) and the LaB6 NW TEM (outlined in black). (115) spot intensity relative to central transmission spot intensity was compared between two electron guns using a line profile. (b) Same comparison about the (224) spot. Comparison about (115) spot showed that the relative intensity ratio between LaB6 NW TEM and W (310) TEM was 2:1 and that about (224) spot is 4:3.

Extended Data Fig. 6 Probe profiles.

(a) 2D electron probe profile simulated using QSTEM based on diffraction and lens aberration conditions of the experiment. Image width: 200pm; (b) A gaussian-shaped beam profile simulating additional probe broadening effect defined as source image size. Image width: 200pm; (c) Theoretical probe profile simulated by convoluting (a) with (b). Image width: 200pm; (d) Line profiles of (a), (b), (c), showing the 87.6pm FWHM of the convoluted probe.

Extended Data Fig. 7 STEM images.

(a) HAADF image taken on a Si (110) specimen using W (310) electron gun. Inset showing the highest resolved spatial frequency of 96pm; (b) Line profile collected from the rectangle-marked region in (a) plotted together with line profiles from LaB6 nanowire STEM image; (c) Raw image data of Fig. 3d.

Extended Data Fig. 8 Stability.

(a) Probe current measured at the objective lens entrance of a SEM equipped with a LaB6 nanowire cold field emission gun with a vacuum level of ~10−8Pa; (b-d) Magnified view of probe current 1 minute profile at 10th, 240th and 480th minute, showing consistent low peak-to-peak noise ratios below 0.5%. Vacuum condition in the TEM electron gun for the LaB6 NW emitter test is in the same level or better than the electron gun shown here. Similar or better performance is assumed for the TEM setup.

Extended Data Fig. 9 Energy spread.

(a) Unmonochromated EELS ZLP comparison among LaB6 nanowire emitter, W (310) emitter and Schottky emitter, used in aberration corrected TEMs. 0.2eV and 0.27 eV are the lowest possible FWHM energy spreads measured for LaB6 nanowire emitter and W (310) emitter in the same JEM-Arm200F TEM; (b) Energy spread versus reduced angular current density plots for the LaB6 nanowire emitter and W (310) emitter; (c) Reduced angular current density plotted against extraction voltage for LaB6 nanowire emitter and W (310) emitter.

Extended Data Fig. 10 Emission area.

(a) Field ion microscopy pattern of (100) apical facet of a LaB6 NW emitter; (b) Field emission microscopy pattern of the same nanowire using the same emitter-screen setup as (a). Red rectangle marking the emission area for (100) facet; (c) Field ion microscopy pattern of the (111) facet of a W needle emitter; (d) Field emission microscopy pattern of the same W needle using the same emitter-screen setup as (c). Red rectangle marking the emission area for (310) facet.

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Zhang, H., Jimbo, Y., Niwata, A. et al. High-endurance micro-engineered LaB6 nanowire electron source for high-resolution electron microscopy. Nat. Nanotechnol. 17, 21–26 (2022). https://doi.org/10.1038/s41565-021-00999-w

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