Atomic-Resolution STEM Imaging of Graphene at Low Voltage of 30 kV with Resolution Enhancement by Using Large Convergence Angle

H. Sawada, T. Sasaki, F. Hosokawa, and K. Suenaga

Phys. Rev. Lett. 114, 166102 – Published 24 April 2015

Abstract

Atomic resolution at a low accelerating voltage with aberration correction is required to reduce the electron irradiation damage in scanning transmission electron microscopy imaging. However, the reduction in resolution caused by the diffraction limit becomes severe with increasing electron wavelength at low accelerating voltages. The developed aberration corrector can compensate for higher-order aberration in scanning transmission electron microscopy to expand the uniform phase angle. The resolution for imaging graphene at 30 kV is evaluated by changing the convergence angle for a probe-forming system with a higher-order aberration corrector. A single-carbon atom on graphene is successfully imaged at atomic resolution with a cold-field emission gun by dark-field imaging at an accelerating voltage of 30 kV.

Received 24 October 2014

DOI:https://doi.org/10.1103/PhysRevLett.114.166102

Authors & Affiliations

H. Sawada^1,*, T. Sasaki¹, F. Hosokawa¹, and K. Suenaga²

¹JEOL Ltd., 3-1-2 Musashino, Akishima, Tokyo 196-8558, Japan
²National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan

^*To whom all correspondence (inquiry) should be addressed. hsawada@jeol.co.jp

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Vol. 114, Iss. 16 — 24 April 2015

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Images

Figure 1
(a) Scheme of the electron path in the Delta system with three dodecapoles, where TL denotes transfer lens, CM denotes condenser mini lens, and OL denotes objective lens. (b) Amplitude $| A_{6} |$ as a function of ${\vec{A}}_{33}$ . The corresponding horizontal axes are $x = | A_{33} | \cos (3 θ_{33 - 31})$ and $y = | A_{33} | \sin (3 θ_{33 - 31})$ , where $3 θ_{33 - 31} = 3 θ_{33} - 3 θ_{31}$ . The value of $A_{32}$ was chosen such that the total threefold astigmatism is zero: ${\vec{A}}_{3_total} = {\vec{A}}_{31} + {\vec{A}}_{32} + {\vec{A}}_{33} = 0$ . The value of $| A_{6} |$ is normalized by assuming that the strength of $| A_{6} |$ at $3 θ_{33 - 31} = 0 °$ with $| A_{31} | = | A_{33} |$ is unity. (c) Ronchigram of the calculated phase at 30 kV in the case of the Delta condition ${\vec{A}}_{3_total} = {\vec{A}}_{31} + {\vec{A}}_{32} + {\vec{A}}_{33} = 0$ , $3 θ_{33 - 31} = 70.5 °$ , and $| A_{31} | = | A_{33} |$ . White to black corresponds to $π$ , and white to white corresponds to $2 π$ . (d) Ronchigram of the calculated phase at 30 kV close to the Delta condition area (i.e., $3 θ_{33 - 31} = 70.5 °$ , $3 / 4 \cdot | A_{31} | = | A_{33} |$ ). (e) Ronchigram of the calculated phase at 30 kV for the third-order spherical aberration corrector of a two-hexapole type.
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Figure 2
(a) Appearance of the developed low-accelerating-voltage microscope Triple C $# 1$ with Delta correctors. (b) Experimental Ronchigram at an accelerating voltage of 30 kV. (c) Calculated phase at 30 kV with sixfold astigmatism of $86 μ m$ . Circle indicates the angle of the $π / 4$ limit, which is 63 mrad. (d) Calculated probe sizes as a function of the convergence semiangle at 30 kV. Defocus is 0 nm, $C_{s} = 0 mm$ , and the sixfold astigmatism $A_{6}$ is $86 μ m$ for a Gaussian probe size of 0 pm. (e) Calculated probe shapes at convergence semiangles of 10, 20, 30, 40, and 50 mrad. The probe was calculated with $4 pm / pixel$ in a real space size and $0.85 mrad / pixel$ on the aperture plane using a $2 k \times 2 k$ Fourier transform. The chromatic aberration, energy spread, Gaussian probe size, defocus, $C_{s}$ , and $A_{6}$ are the same as those used in the previous probe calculation.
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Figure 3
Simulated annular dark-field (ADF) STEM images of graphene at convergence semiangles of (a) 15, (b) 20, (c) 25, (d) 30, (e) 35, (f) 40, (g) 45, and (h) 50 at 30 kV. A chromatic aberration of 0.61 mm and an energy spread of 0.4 eV were taken into account in all the images. Defocus is 0 nm, $C_{s} = 0 mm$ , and the sixfold astigmatism $A_{6}$ is $86 μ m$ with a Gaussian probe size of 50 pm. The specimen has a single atomic layer of carbons in this calculation. The detection angle for the low-angle annular dark-field (LAADF) image ranged from ( $α + 10$ ) mrad to $(α + 10) \times 4 mrad$ in the simulation, where $α$ denotes a convergence semiangle.
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Figure 4
Raw data for high-resolution DF images using graphene [21] for convergence angles of (a) 22, (b) 26, (c) 30, (d) 32, (e) 38, (f) 40, and (g) 48 mrad. The detection angle for the LAADF image ranges from $(α + 10) mrad$ to 200 mrad in the observation.
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Figure 5
(a) Gaussian fitting from carbon atomic images at each convergence semiangle. (b) FWHM of the carbon atom image by Gaussian fitting.
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Figure 6
(a) LAADF image of graphene at 30 kV. Lower right corner shows simulated image with a convergence angle of 48 mrad and Gaussian probe size of 50 pm. (b) Calculated probe shape with Gaussian probes of 0 and 50 pm. Probe dimensions are (FWHM) 92.8 pm, D50: 167.5 pm, and D59: 210.5 pm. (c) A Fourier transform of the STEM image.
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