Photon Factory Activity Report 2018 #36 (2019)

BL-7C/ 2018G066

Detecting X-ray induced change in the tip-surface force on Ge by XANAM
Shushi SUZUKI*1, Shingo MUKAI2, Wang Jae CHUN4, Masaharu NOMURA3,and Kiyotaka ASAKURA2
1
Nagoya University, Nagoya, Aichi 464-8603, Japan
2
Hokkaido University, Sapporo, Hokkaido 001-0021, Japan
3
KEK-PF, Tsukuba, Ibaraki 305-0801, Japan
4
International Christian University, Mitaka, Tokyo 181-8585, Japan
1. Introduction
In recent years, with the progress in research on theapplication of single atoms and nanoclusters to electronicdevices, the demand for surface / interface elemental
analysis at the nanoscale has increased. We havedeveloped X-ray aided noncontact atomic force
microscopy (XANAM), a scanning probe microscopy
combined with synchrotron X-ray designed for elemental
analysis at the nanoscale. We reported that Au and Ni
surfaces could be identified by detecting the force change
in the tip-surface interaction induced by X-ray absorption,
which occurs at those element’s edge energy [1]. Based on
the NC-AFM experimental and theoretical studies, it iswell-known that the tip-surface force includes covalentbond force in addition to van der Waals force, electrostaticforce. We expected that if the electron density of theorbital in the covalent bond changed with X-ray core-level
electron excitation, it could be used for the elemental
analysis on an atom beneath the tip from the X-ray
absorption edge energy (Fig. 1). By extracting the change
in the covalent bond force, the atomic resolution could beachievable. Therefore, the force spectrum of the NC-AFM(interatomic force dependence concerning the tip-sample
separation) was measured, and component analysis of theforce was performed by caculations based on the theory.
As a result, it was found that force components in thechange could be classified into the electrostatic force andthe covalent bond force. However, the direct proof
showing the involvement of covalent forces is still
ambiguous, because there have been no experimental
results on semiconductor surfaces except the metals

Fig. 1 Basic concept of XANAM
mentioned above. Here, we report that the spectroscopic

measurements on a Ge surface. It is well known that theNC-AFM tip can intera ct with the surface atoms on thesemiconductor surface with the covalent bonding; thus weexpected to detect of the changes in the force induced byX-ray at the Ge-K absorption edge energy, resulting in thedirect proof of the issue.
2. Experimental
The experiments were performed at BL-7C of thePhoton Factory, Institute of Material Structure Science,High Energy Accelerator Organization (KEK-IMS-PF). A
non-contact atomic force microscope was operated in ahome-build ultrahigh vacuum (UHV) chamber equippedwith two beryllium windows for X-ray penetration
through the chamber. The microscope stage had four kindsof freedom to set the sample surface on the X-ray pass: X-,
Y- and Z- movements and a rotation around the vertical(Z) axis. We focused an X-ray beam on the sample
position by a sagittal focusing of Si(111) double-crystal
monochromator.
As a sample, a Ge(001) single crystal was used as asample. The sample surface was cleaned before use by
sputtering and annealing [2]. An AFM-probe wasfabricated by using a quartz tuning fork andelectrochemically-etched tiny tungsten (W) tip with thesimilar configuration of the qPlus sensor [3]. The typicalresonance frequency of the sensor was ~30 kHz. As theforce signal, frequency shift (Δf) from the resonancefrequency was recorded and converted to the values offorce using the Sader-Jarvis equation [4]. All themeasurements were carried out with irradiating the X-rays
with the energies around the Ge-K absorption edge. The
data was recorded by Nanonis control systems (Nanonis,SPECS Zurich GmbH).
3. Results and Discussion
Fig. 2(a) shows a result obtained by force spectroscopy
measurements with scanning X-ray energy around the GeK edge absorption edge. The vertical and horizontal axes
are the tip-surface separation and X-ray energy,respectively. The color contrast indicates the strength ofthe force detected by the tip. For instance, the force from-5 to -8 nN was observed as the color range from green toPhoton Factory Activity Report 2018 #36 (2019)

We have developed XANAM to obtain elemental
information on surfaces at the nanoscale using NC-AFM
combined with SR X-ray. To obtain the direct proof of theforce change surely involving the covalent bond force, weadopted the force spectroscopy measurements to Ge(001)with scanning X-ray energy around the Ge-K edge
absorption edge. As a result, we observed the X-ray
induced changes as the small gap over the gradualbackground change. We concluded tentatively the gap
presumably related to the X-ray induced change in thecovalent bond force.
Fig. 2 A set of force spectra on Ge(001) measured
with scanning X-ray energy around the Ge-K edge
absorption edge. The red curve is the absorption
spectrum of Ge, separately-measured.
red via yellow, as shown in the bar graph at the right side.
The negative force means the attractive force in the figure.
Besides, the X-ray absorption spectrum of Ge-K in thesame energy range is superimposed on the figure as areference, which was separately obtained in thetransmission mode using a thin film (3 μm) sample madeof Ge. The energy of 11103 eV is used as the absorption
edge energy of Ge-K in this report.
First, at the energies below the X-ray absorption edge,
the shape of the force curve was almost maintained. The
magnitude of the attractive force was kept around -7 ~ -8nN. Then, the force curve started to change its shape fromthe absorption edge energy with decreasing the attractiveforce.
By analyzing the force curve change, a small gap was
found near the X-ray absorption edge energy. The state of
the gap could be considered as the change-induced byelectron excitation from the core level electron to theelectron states of the covalent bond between the tip andthe surface. Thus, we tentatively concluded that thechange restricted to the absorption edge energy wasobserved experimentally as the covalent bond force.
However, the background of the change decreased
gradually, too. It seemed to cover the net change of thegap. Although we cared for the measurements withenough sensitivity to the covalent bond, the current resultshows that detecting X-ray induced electrostatic forces
was still inevitable, because the force involves van der
Waals force and electrostatic force. For increasing thesensitivity to such energetically-limited change, it isimportant to understand the background changes. Thus, itis necessary to analyze the force components to attributethe origins of the changes in the force curves as the nextstep.
4. Conclusion

Acknowledgments
This work was partially supported by Tatematsu
Foundation, as well as the previous grants from the JSPS,JST, and NEDO. The experiments were carried out under
the approval of Photon Factory Advisory Committee(PAC: Proposal No. 2012G099, 2014G114, 2016G109,2018G066).
References[1] S. Suzuki, Y. Koike, K. Fujikawa, W.-J. Chun, M.
Nomura, and K. Asakura , Chem. Lett., 33, 636-637,2004.; S. Suzuki, Y. Koike, K. Fujikawa, N.
Matsudaira, M. Nakamura, W.-J. Chun, M. Nomura,and K. Asakura, Catal. Today, 117, 80-83, 2006.; S.
Suzuki, M. Nakamura, K. Kinoshita, Y. Koike, K.
Fujikawa, N. Matsudaira , W.-J. Chun, M. Nomura,and K. Asakura, J. Phys., 61, 1171-1121, 2007.;
Suzuki, S., Bull. Chem. Soc. Jpn., 88, 240-250,2015.
[2] S. Godlewski et al., Phys. Rev., B 92 (2015)115403.
[3] F. J. Giessibl, Appl. Phys. Lett. 76, 1470-1472,
2000.
[4] J. E. Sader and S. P. Jarvis, Appl. Phys. Lett., 84,1801-1803, 2004.
*shushi@chembio.nagoya-u.ac.jp