Photon Factory Activity Report 2016 #34 (2017)

BL-7C/ 2016G109

Improvements in data acquisition and processing toward XANAM imaging
Shushi SUZUKI*1, Shingo MUKAI2, Wang Jae CHUN4, Masaharu NOMURA3,and Kiyotaka ASAKURA2
1Nagoya 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
Scanning probe microscopy (SPM) provides structural
information about surfaces on flat substrates. In addition,
as a spectroscopy using a probe, it is also used foranalysis of surface physical properties and chemical
properties. Especially, for the noncontact atomic force
microscopy (NC-AFM), chemical information can bealso obtained by force spectroscopy with theoreticalanalysis of the force components between the tip and thesample surface [1]. The method excellently showed a
new possibility of the SPM and NC-AFM. To make iteasier-to-use, we proposed a methodology of NC-AFM
combined with synchrotron radiation (SR) X-ray toobtain data for chemical information on the surfaces
using fundamental knowledge of X-ray elemental
analysis, named "X-ray aided noncontact atomic force
microscopy (XANAM)". Using the SR X-ray, the corelevel electrons can be excited at energies of X-ray
absorption edge for the sample surfaces. On the other
hand, the presence of covalent bonding between the tip
and the sample surface is commonly well-known. From
the those, we proposed that the covalent bonding createdbetween the tip and the sample surface can be used forthe detection. If the electron excitation alters electron
density in the covalent bond, the force between the tip
and the surface can be changed, i.e., the electron
excitation is detected by the AFM-tip, the atom species

Fig. 1 Basic concept of XANAM

directly beneath the tip can be identified (Fig. 1). In theprevious reports [2], X-ray energy dependency of the ona gold (Au) partially deposited silicon (Si) surface
showed the clear different spectral features on Au and Si
regions near the Au-L3 absorption edge energy. Besides,
we confirmed the existence of covalent bonding force
and found its strength was weakened by X-rays withenergy near the Au - L 3 absorption edge. Although theabove method was a spectroscopy on a fixed point on thesurface, we also reported that it could be extended to twodimensions, providing the results comparable to the NCAFM image data on the surface. However, theprocedures have not been well-established. In this report,
recent progress in the improvement of experimental
procedures for XANAM imaging.
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 fourkinds of freedom to set the sample surface on the X-ray
pass: X-, Y- and Z- movements and a rotation aroundvertical (Z) axis. We focused an X-ray beam on thesample position by a sagittal focusing of Si(111) doublecrystal monochromator. In this measurement, a Au
deposited substrate was used as a sample. An AFMprobe was fabricated by using a quartz tuning fork and anelectrochemically-etched tiny tungsten (W) tip with thesimilar configuration of the qPlus sensor [3]. The typicalresonance frequency of the sensor was ~29.3 kHz,
respectively. As the force signal, frequency shift (Δf)
from the resonance frequency was recorded. All themeasurements were carried out with irradiating the X-ray
beam with energies around the Au-L3 absorption edgeenergy. The data was recorded by Nanonis control
systems (SPECS Zurich GmbH). The data was processedby the programs coded by Mathematica language(Wolfram).

Photon Factory Activity Report 2016 #34 (2017)

3. Results and Discussion
Fig. 2(a) and (b) show results Δf on the surface under
X-ray irradiation tuned at 11869 eV and 11923 eV,
respectively. To obtain these data, the force spectroscopy
measurements, the distance dependency of Δf wasrecorded in two-dimensionally on a Au deposited part in
the same region. Because the Au region was a depositedpart, it’s height was higher than that of the underlying Siregion. The height difference was corrected by shifting
the data origin in each force spectrum. In this case, theorigin was defined as the point where the amplitude of
the sensor began to decrease in each spectrum. Hence,
the height difference between Au and Si regions could beapparently cancelled. However, distinctive contrast
differences between two Δf images were observed in therange of -45 ~ -17 Hz. The lower values indicatedstronger attractive force. The contrast of Au region in Fig.
2(a) was brighter than that of Si region, while thecontrasts in two regions became ambiguous in Fig. 2(b)as much as the Fig. 2(a). The X-ray energy for the former
and the latter cases were 11869 eV and 11923 eV,
respectively. Because the change in the contrasts ofimages appeared around the Au-L3 absorption edge of11919 eV, it was proved that the Au regions wereidentified by XANAM using the Au-L3 absorption edgeenergy. At this stage, because the changes in a submicrometer scale of the Au region could be observed
clearly, several hundred of spatial resolution could beestimated as the microscopic usage of XANAM. To
obtain further clearer contrast changes, surface structural
factors of height difference should be eliminated more
precisely for estimation of the spatial resolution.
4. Conclusion

Fig. 2 XANAM images obtained by force
spectroscopy measurements on the partially Audeposited Si substrate surface under X-ray
irradiation with the X-ray energy of (a) 11869 eVand (b) 11923 eV. The frequency shift data wererecorded in the area of 1×2 μm2.

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-rayirradiation. Comparing with the two images with two Xray energies of 11869 eV and 11923 eV, theforce changes in the Au region was imaged withseveral hundred of spatial resolution.
Acknowledgements
This work was partially supported by Murata Science
Foundation and Tatematsu Foundation, as well as theprevious grants from the JSPS, JST, and NEDO. The
experiments were carried out under the approval ofPhoton Factory Advisory Committee (PAC: Proposal No.
2012G099, 2014G114, and 2016G109).
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*shushi@chembio.nagoya-u.ac.jp