Investigating ultraflexible freestanding graphene by scanning tunneling microscopy and spectroscopy

R. Breitwieser, Yu-Cheng Hu, Yen Cheng Chao, Yi Ren Tzeng, Sz-Chian Liou, Keng Ching Lin, Chih Wei Chen, and Woei Wu Pai

Phys. Rev. B 96, 085433 – Published 24 August 2017

Abstract

A strictly two-dimensional (2D) material such as freestanding graphene (FSG) is rarely investigated at the atomic scale by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). A basic difficulty in probing FSG by STM and STS is the mechanical instability when a highly compliant 2D atomic layer interacts with a proximal tip. Here we report a detailed method to conduct reliable STM and STS on FSG with atomic precision. We found that FSG is intrinsically rippled and exhibits a nonlinear strain-stress relation under applied normal forces; it shows a very soft region of bending strain and stiffer regions of in-plane tensile strain once the nanoscale ripples of FSG are eliminated. The elimination of the nanoripples can be controlled by tip-induced pulling or pushing force through the so-called closed-loop Z-V STS mode which can monitor the FSG deformation. A key factor for controllable STM and STS measurements is to select tunneling set points to place FSG in metastable configurations, as determined from stress-strain (i.e., Z-V) response. Atomic imaging and electronic states thus measured must be interpreted by considering the dynamical deformation of FSG as tunneling parameters, and therefore tip-FSG forces, are varied.

Received 16 November 2016
Revised 6 June 2017
Corrected 6 September 2017

DOI:https://doi.org/10.1103/PhysRevB.96.085433

Physics Subject Headings (PhySH)

Graphene

Condensed Matter, Materials & Applied Physics

Corrections

6 September 2017

Erratum

Publisher's Note: Investigating ultraflexible freestanding graphene by scanning tunneling microscopy and spectroscopy [Phys. Rev. B 96, 085433 (2017)]

R. Breitwieser, Yu-Cheng Hu, Yen Cheng Chao, Yi Ren Tzeng, Sz-Chian Liou, Keng Ching Lin, Chih Wei Chen, and Woei Wu Pai

Phys. Rev. B 96, 119903 (2017)

Authors & Affiliations

R. Breitwieser^1,*, Yu-Cheng Hu¹, Yen Cheng Chao¹, Yi Ren Tzeng², Sz-Chian Liou¹, Keng Ching Lin³, Chih Wei Chen¹, and Woei Wu Pai^1,4,†

¹Center for Condensed Matter Sciences, National Taiwan University, Taipei 106, Taiwan, Republic of China
²Institute of Nuclear Energy Research, Long Tan, Taoyuan 325, Taiwan, Republic of China
³Department of Physics, Catholic Fu Jen University, Taipei 242, Taiwan, Republic of China
⁴Department of Physics, National Taiwan University, Taipei 106, Taiwan, Republic of China

^*Corresponding author: romain.breitwieser@upmc.fr
^†Corresponding author: wpai@ntu.edu.tw

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Vol. 96, Iss. 8 — 15 August 2017

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Images

Figure 1
(a) SEM image (beam energy: 20 keV) of a single-layer FSG on a Mo-TEM grid (300 mesh/inch). (b) Z-V measurements (closed-loop feedback) on single-layer FSG ( $I_{t} = 2.0 nA$ , $V_{b} = 0.8 V$ ) showing a typical electromechanical response of a FSG subjected to STM tip-induced forces, i.e., electrostatic ( $F_{e}$ ) and van der Waals ( $F_{vdW}$ ) forces. (c) Schematics of the FSG's atomic structure under tip-induced deformation illustrating the pulling (up), the nanoripples flipping (middle), and the pushing (down) effects through $F_{e}$ and $F_{vdW}$ . For largely deformed FSG, the C-C bond stretching gives rise to a restoring force $(F_{r})$ in graphene.
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Figure 2
(a)-(c) Atom-resolved STM images of single-layer FSG in different configurations: (a) Pushed FSG at the Z-V lower plateau ( $I_{t} = 0.3 nA, V_{b} = 0.2 V, 8 \times 8 {nm}^{2}$ ); (b) Pristine rippled FSG at the Z-V midpoint ( $I_{t} = 0.6 nA, V_{b} = 0.8 V, 10 \times 10 {nm}^{2}$ ); (c) Pulled FSG at the upper plateau ( $I_{t} = 0.7 nA, V_{b} = 0.3 V, 8 \times 8 {nm}^{2}$ ). (d) Line profiles from Figs. 2–2. (e) Simulation of the apparent atomic corrugation of single-layer FSG due to tip-induced mechanical deformation by van der Waals interaction. (f) The lattice and the line (L) for plotting corrugation. (g) Profile A for an average tip-FSG distance of $3.6 Å$ and $4.1 Å$ for profile B [A, B also marked on the $u + d$ vs d curve in (e)]. Profile C is an assumed corrugation for FSG without tip interaction.
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Figure 3
Raw atom-resolved STM images of single-layer FSG showing (a) the honeycomb lattice of carbon atoms, (b) tip-condition change (arrow marked) and concomitant apparent switching from triangular to honeycomb lattice, (c) coexisting triangular and honeycomb lattices (dashed and solid circles, respectively) on a rippled FSG, and (d) line profiles from triangular and honeycomb lattices of (c).
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Figure 4
(a) Z-V measurements (closed-loop feedback) and corresponding I-V spectroscopy curves (open-loop feedback) executed for FSG in different configurations, i.e., with different set points ( $I_{t} = 0.7 nA$ and $V_{b} = 2.0$ , 0.5, 0.4, 0.3, 0.1 V). (b) I-V measurements executed with a constant gap resistance $(2 G Ω)$ for FSG initially pushed (0.1 nA, 0.2 V) and pulled (1.0 nA, 2.0 V).
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Figure 5
A typical Z-V spectrum and raw atom-resolved STM image ( $I_{t} = 0.6 nA$ , $V_{b} = 0.8 V$ ) of a bi-layer FSG.
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Figure 6
(a) STM image ( $I_{t} = 0.53 nA$ , $V_{b} = 0.7 V$ ) and (b) Z-V spectroscopy of graphene on copper obtained in a flat area. The inset is the corresponding differential curve of dZ/dV-V. (c) STM image ( $I_{t} = 0.53 nA$ , $V_{b} = 0.7 V$ ) and (d) Z-V spectroscopy of freestanding graphene on copper obtained in a more corrugated area.
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