Photon Factory Activity Report 2017 #35 (2018)

BL-7C/ 2016G109

Measurements of 2D force map on Ni nanoparticles on HOPG for XANAM imaging
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
X-ray aided noncontact atomic force microscopy(XANAM) is a scanning probe microscopy combined withsynchrotron X-ray designed for elemental analysis in thenanoscale. We reported previously on Au and Ni surfaces
could be identified by detecting the force change in the tipsurface interaction induced by X-ray absorption which
occurs at those element’s edge energy [1]. From theprevious NC-AFM experimental and theoretical studies, itis well-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 with respect to the tipsample separation) was measured, and component
analysis of the force was performed. As a result, it was
found that force components in the change could beclassified into the electrostatic force and the covalent bondforce. Furthermore, we examined the establishment ofimaging procedure for the elemental mapping at thenanoscale, proposed as the extended-force spectroscopy in
two dimensions with X-ray energy scanning. Previously,

Fig. 1 Basic concept of XANAM

X-ray energy dependency of the force on a gold (Au)partially deposited silicon (Si) surface was recorded
successfully. Although the spatial resolution of theobtained image was not so high, it could be regarded thatthe procedure was established as one of the imaging
technique for XANAM. Here, as the remaining problem,
we have pursued how the imaging procedure could be
optimized for XANAM using a sample of Ni nanoparticles
deposited on an HOPG substrate.
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 vertical (Z)axis. We focused an X-ray beam on the sample position
by a sagittal focusing of Si(111) double-crystal
monochromator. In this measurement, a Ni deposited
HOPG substrate was used as a sample. An AFM-probe
was fabricated by using a quartz tuning fork andelectrochemically-etched tiny tungsten (W) tip with thesimilar configuration of the qPlus sensor [2]. 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 [3]. All themeasurements were carried out with irradiating the X-rays
at the Ni-K absorption edge energy. The data was recorded
by Nanonis control systems (Nanonis, SPECS ZurichGmbH).
3. Results and Discussion
Fig. 2(a) and (b) show results obtained from XANAM
imaging. Fig. 2(a) shows a contrast difference of force
calculated from the force spectra measured in each pixel.
The values in Fig. 2(a) were corresponded to the values atthe same distance from the sample surface in the force

Photon Factory Activity Report 2017 #35 (2018)

spectra (The red crosses in A and B, as shown in the rightcolumn). The height differences on the nanoparticles andbare regions of HOPG were corrected using thetopographical information of the nanoparticles, resulting
in the cancelation of the origin of the separation in Fig.
2(a). The image in Fig. 2(b) was an NC-AFM image ofthe same area obtained in Fig. 2(a), showing the Ni
nanoparticles formed on an HOPG surface. Comparedwith two images of Fig. 2(a) and (b), the contrasts in Fig.
2(a) shows the similar tendencies to the corrugations ofnanoparticles in Fig. 2(b). Thus, it was confirmed that the2D force map in Fig. 2(a) was obtained for the area
observed in Fig. 2(b).
In this trial, Fig. 2(a) only have 256 (= 16x16) pixels,
which corresponds to 6.25 nm / pixel. Because the timeinterval of measuring force spectra was ~10 s, the time ofscanning a frame was 43 min for the 2D force map. To
evaluate how far the spatial resolution could be obtained,
it was necessary to increase the number of pixels andshorten individual force spectrum measurement time,
although another factor of X-ray energy scan should beconsidered. Further overall improvements of themeasurements are in progress to evaluate the spatial
resolution of XANAM imaging with X-ray energy scan.

enough to conclude the spatial resolution, the sametendencies of image contrasts were obtained on the Ni
nanoparticles on the HOPG surface.
Acknowledgements
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, and2016G109).
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] F. J. Giessibl, Appl. Phys. Lett. 76, 1470-1472,2000.
[3] J. E. Sader and S. P. Jarvis, Appl. Phys. Lett., 84,1801-1803, 2004.
*shushi@chembio.nagoya-u.ac.jp

4. Conclusion
We have developed XANAM to obtain elemental
information on surfaces at the nanoscale using NC-AFM
combined with SR X-ray. To obtain image data ofXANAM for estimation of the spatial resolution, weadopted the two-dimensional force spectroscopy
measurements as XANAM imaging under X-ray
irradiation. Although the number of pixels were not