In Situ Reproducible Sharp Tips for Atomic Force Microscopy

Jo Onoda, Tsuyoshi Hasegawa, and Yoshiaki Sugimoto

Phys. Rev. Applied 15, 034079 – Published 26 March 2021

Abstract

Atomically sharp tips are a requirement for scanning-probe microscopy, such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Compared with STM, AFM imaging is more sensitive to the sharpness of tip apices because long-range forces act as a background signal on the high-resolution AFM images originating from short-range forces. Here we report the investigation of in situ reproducible sharp tips for AFM. We make an ${Ag}_{2} S$ crystal, a mixed ionic and electronic conductor, on a conventional $Si$ cantilever, and controllably grow and shrink the $Ag$ nanoprotrusion by changing the polarity of the bias voltage between the tip and the sample. We are able to reduce the contribution of long-range forces by growing a $Ag$ nanoprotrusion on the ${Ag}_{2} S$ tip, and obtain atomic-resolution AFM images. We also confirm that the ${Ag}_{2} S$ tip with a $Ag$ nanoprotrusion, the end of which presumably terminates in $Si$ atoms, is capable of simultaneous AFM and STM measurements.

Received 29 December 2020
Revised 2 February 2021
Accepted 2 March 2021

DOI:https://doi.org/10.1103/PhysRevApplied.15.034079

Physics Subject Headings (PhySH)

Nanostructures

Noncontact atomic force microscopy Sample preparation Scanning probe microscopy Scanning tunneling microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Jo Onoda ^1,*, Tsuyoshi Hasegawa², and Yoshiaki Sugimoto ³

¹Department of Physics, University of Alberta, Edmonton, Alberta T6G 2J1, Canada
²Department of Applied Physics, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan
³Department of Advanced Materials Science, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8561, Japan

^*jonoda@ualberta.ca

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Issue

Vol. 15, Iss. 3 — March 2021

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Images

Figure 1
Mechanisms of growth and shrinkage of the $Ag$ nanoprotrusion. (a) When a negative bias voltage is applied to the sample, reduction of ${Ag}^{+}$ cations proceeds and $Ag$ atoms are precipitated on the surface of the ${Ag}_{2} S$ base. (b) When a positive bias voltage is applied to the sample, ${Ag}^{+}$ cations migrate toward the bottom of the ${Ag}_{2} S$ base, which induces the oxidation of $Ag$ atoms on the surface.
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Figure 2
Tip-height change during the growth and shrinkage of a $Ag$ nanoprotrusion on the clean $Ag$ (111) surface. (a) Growth mode with $I_{t} = 10 nA$ and $V_{S} = - 4 V$ . The set point is changed to 15 nA at around 3000 s (blue line). (b) Shrinkage mode with $I_{t} = 10 nA$ and $V_{S} = + 4 V$ . The experiments for (a),(b) are conducted in constant-current STM mode without cantilever oscillation.
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Figure 3
$Δ f (z)$ curves measured on the $Si$ (111)-( $7 \times 7$ ) surface with various states of a ${Ag}_{2} S$ tip: state 1, after intentional crash of the tip into the surface; state 2, after the first growth of a $Ag$ nanoprotrusion; state 3, after the shrinkage of the nanoprotrusion; state 4, after the second growth of the nanoprotrusion. The origin of the graph describes the divergence point of a fitted $Δ f (z)$ curve for State 1 (not shown). The acquisition parameters are the resonance frequency $f_{0} = 142.818 kHz$ , the cantilever-oscillation amplitude $A = 145 Å$ , the spring constant $k = 22.8 N/m$ , and $V_{S} = - 100 mV$ .
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Figure 4
Force-spectroscopy data for the $Si$ (111)-( $7 \times 7$ ) surface with a ${Ag}_{2} S$ tip with and without a $Ag$ nanoprotrusion. (a) $Δ f (z)$ curves measured above the $Si$ adatom (white arrow in the upper inset) with a $Ag$ nanoprotrusion (red curve) and without the protrusion (black curve). The image in the upper inset is taken with the set point of $Δ f = - 24.4$ Hz, represented by the green arrow on the red curve, while the image in the lower inset is obtained with the set point of $Δ f = - 99.1$ Hz, indicated by the orange arrow on the black curve. (b) $F (z)$ curves converted from the $Δ f (z)$ curves in (a). The long-range force (black curve) is fitted with $F_{vdW} (z)$ (blue curve). The inset in (b) shows the short-range-force curve measured on the $Si$ adatom. The origins of the graphs describe the divergence point of the long-range force. The acquisition parameters are $f_{0} = 150.618$ kHz, $A = 173 Å$ , $k = 26.7$ N/m, and $V_{S} = - 200$ mV.
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Figure 5
Simultaneously obtained force and current spectroscopic data. (a) $Δ f (z)$ (red) and $⟨ I_{t} (z) ⟩$ (black) curves measured on the $Si$ adatom in Fig. 4. (b) $F (z)$ and $I_{t} (z)$ (black) curves converted from the $Δ f (z)$ and $⟨ I_{t} (z) ⟩$ curves in (a). The $F (z)$ curve is deconvoluted to a short-range-force ( $F_{SR}$ ) curve (solid red line) and a long-range-force ( $F_{LR}$ ) curve (dotted blue line). AFM (c) and STM (d) images are simultaneously obtained with the AFM topographic mode [the set point of $Δ f = - 14.7$ Hz is represented by the blue arrow on the red curve in Fig. 4]. The acquisition parameters as the same as for Fig. 4.
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Figure 6
Cauchy plot for ${Ag}_{2} S$ . The red line represents the best-fitting line.
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