Photon Factory Activity Report 2014 #32 (2015) B

BL-7C/2014G114

A challenge toward surface elemental mapping by 3D XANAM spectroscopy
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. Instructions
Scanning probe microscopy (SPM) is one of thepowerful techniques to obtain information on surface
structures on solid substrate surfaces. In addition to thefunction, spectroscopic usages of SPM are continuously
developed for pursuing physical and chemical properties
of surfaces/interfaces at the nanoscale. Especially, force
spectroscopy in noncontact atomic force microscopy (NCAFM) measurements is a powerful tool to reveal
fundamental aspects of long range and short range
physical and chemical interactions. In 2007, throughprecise measurements on difference in the covalentbonding force between a tip and a surface, elemental
species on individual atoms observed in NC-AFM images
were distinguished on a silicon surface with two otherelemental species [1].
The above spectroscopy study does not use Synchrotron
radiation (SR) X-ray and proved that the chemicalsensitivity can achieve atomic level by means of SPM tips,
although it requires a specialized setting to observe
surfaces with atomic resolution on a flat surfaces. On theother hand, there are many techniques on elemental
analysis developed in the X-ray experiments, such as XPS,XRF, and XAFS. Basics on them for elemental analysis
are recognizing energy in relation to excitation of corelevel electrons, emission of photoelectrons and fluorescent
X-ray. The techniques are applicable to samples withFig. 1 Basic concept of XANAM

various morphologies, for instance, a surface with humps.
If such flexibility in the applicability can be implemented
into NC-AFM and force spectroscopy, more versatile
usages, which are accessible to fabrication ofsemiconductors, catalysts, and sensors, are expected.
On this point, our study on X-ray aided noncontact
atomic force microscopy (XANAM) [2-4] is potentially
expected to provide a possible solution for the issue on
applications to non-ideally flat surfaces. To the present, acustomized NC-AFM for SR X-ray experiments wasdeveloped and was tuned for the experiments on somelithographed samples, nanomaterials-fixed surfaces, etc.
Recently, the probe system on this apparatus has beenrefined to a new one adopting a qPlus sensor type probe[5]. The qPlus sensor type probe is very sensitive to thecovalent bonding force, or chemical interactions whichappears in force spectra, as well as van der Waals force
and electrostatic force interactions. Thus, we proposedthat HOMO/LUMO states localized at the covalent bondplay a role of an extremely small detector of electron
excitation from the core-level states of a sample surface
when the surface was irradiated with the SR X-ray tuned
at the absorption edge energy of the elements beneath thetip. Detecting the excitation of electrons at certain X-ray
energy by the tip, we expected to obtain an elemental orchemical map of the sample surface with structural
information at the nanoscale (Fig. 1). Actually, force
spectra of a gold (Au) partially deposited silicon (Si)surface showed two distinctively-different spectralfeatures on Au and Si regions by irradiating with X-ray
tuned at Au L3 absorption edge energy. Near the edgeenergy region, strength of overall forces reduced
compared to the one around the near-edge energy region.
Although the spectra should be treated theoretically todivide force components [6, 7], the performance of theXANAM apparatus achieved a certain level of providing
sufficient energy dependencies of force spectra forelemental analyses. Thus, we examined to expand theforce spectroscopy in two dimensions as an XANAM
imaging technique. Three dimensional data matrix for theforce along the horizontal and vertical directions of near
the surface with an additional axis of incident X-ray

Photon Factory Activity Report 2014 #32 (2015) B

energy was obtained. In this report, we show a concept
how the XANAM images could be measured and atentative data on the Au-partially deposited Si surface.
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 Au partiallydeposited substrate was used as a sample. A qPlus sensor
was fabricated by using a quartz tuning fork and anelectrochemically-etched tiny tungsten (W) tip. A prongof the fork was fixed on a cantilever holder, followed bysticking the W tip on the other side of the prong withwiring for connecting to a current-to-voltage converter(DLPCA200, FEMTO Messtechnik GmbH) in order tomeasure the current signal. In this case, the qPlus sensor
was oscillated by applying the ac voltage to electrodes on
the prongs directly. Typical Q value and resonancefrequency of a fabricated qPlus sensor were ~500 and~27.5 kHz, respectively. All the measurements werecarried out with irradiating the X-ray beam tuned aroundthe Au L3 absorption edge energy. The data was recorded
by Nanonis control systems (SPECS Zurich GmbH).
Three dimensional images were carried out by repeating
the force spectroscopy measurements on virtual grid
points in two dimensions on the sample surface. An areaof 2×2 μm2 was divided into 64×64 points of the grid inthis experiments. As a result, the 3D data matrix of theforce was obtained along two horizontal and one verticaldirections on the sample surface.
3. Results and Discussion
The performance of the qPlus sensor was confirmed in
respect to signal to noise ratio for the measurements onfrequency shift (Δf), amplitude, phase, excitation, andelectric current. Sharpness of an apex of the tip was very
important for suppressing the level of background signal
related to the electrostatic force. Generally in the case of
normal operation without irradiation with light, influenceof the electrostatic force can be reduced by choosing
certain bias voltage between the tip and the surface bycompensating the contact potential difference (CPD)
between them. Besides, previous paper showed aFig. 2 XANAM (NC-AFM) Images obtained by force
spectroscopy measurements on 64×64 grid points on
2×2 μm2 of a partially Au-deposited Si(Ox) surface.
(a): Topography, 11869 eV, (b): frequency shift,11869 eV, (c): Topography, 11923 eV, (d): frequency
shift, 11923 eV.
sharpened tip is fundamentally-insensitive to theelectrostatic force, compared to a rounded-shaped tip [8].
However, under X-ray irradiation condition, the X-ray
induces positive and negative charge states at random inboth of the tip and the sample, and varied the CPD withenergy scanning. Since it is very difficult to reduce those
influence properly, sharper tip is more profitable to obtaindata. Actually, although the influence of the inducedelectrostatic force could not be eliminated completely inmost measurements in this term, some data weresuccessfully obtained with reducing the influence.
Fig. 2 shows results of (a) topography and (b) Δf
obtained by repeating the force spectroscopy
measurements on the virtual girds under X-ray irradiation
tuned at 11869 eV. Besides, Fig. 2(c) and (d) were theabove signals obtained in the same area under X-ray
irradiation tuned at 11923 eV. The Δf images were a sliceof the 3D data matrix at certain tip position from thesurface. The surface position was defined as a position
where decay of the oscillation amplitude of qPlus sensor
begun. At an intersection of lines in each image, the tip
position is approximately 0.5 nm far from the Au surface.
Besides, image contrast are adjusted to be common
between a pair of each signal. In the image, a squareshaped Au deposited region, a bare naturally-oxidized Si
area, and unknown garbage adsorbed on the surface before
starting the measurements. These species were labelled as“Au”, “Si”, and “g”, respectively. Besides, a guide for thePhoton Factory Activity Report 2014 #32 (2015) B

Au region was described as a white square with dottedlines in Fig. 2(a) and (c). In these images, morphologies in
the Au region and the garbage were observed in a similarfashion. The fact indicates the tip-shape on the qPlus
sensor was maintained during the measurements, andguaranteed the quality of data on Δf images in Fig. 2(b)and (d).
Comparing the two images in Fig. 2(b) and (d),morphologies in the Au region were observed in adifferent manner, while the garbage were observed
similarly in both images. Since the topographical change
were not observable as mentioned above, the difference inthe Δf images can be interpreted as X-ray induced
phenomenon detected in two dimensions. In accordancewith the previous results [9], the force interaction wereweakened by X-ray near the absorption edge region. In
this experiments, X-ray energies of 11869 eV and 11923
eV correspond to lower and higher energies than the Au
L3 absorption edge energy of 11919 eV, respectively.
Thus, the force weakening possibly occurred and thecontrast formation in the Au region might be disrupted inFig. 2(d). No such variations were found in the bare Si
region and the garbage. Consequently, the presence of Au
atoms were visualized directly in two dimensions.
4. Conclusion
A challenge to obtain elemental maps has been carriedout by the grid measurements of continuous force
spectroscopy in two dimensions on the sample surface. By
irradiating the X-ray tuned near the Au L3 absorption edgeenergy, weakening the force signal in a square Au region
was visualized in two dimensions. On the contrary, thesignal in other parts were not changed. Although further
examinations for other X-ray energies and influence ofthickness of the Au layer are needed to consider the spatial
resolution of XANAM, the results proved the availabilityof two dimensional force spectroscopy combined with Xray irradiation for elemental mapping on individual
structures on surfaces.
Acknowledgements
This work is supported by a Grant-in-Aid for Scientific
Research (C) from the JSPS (Number: 25390079) and ispartially supported by Grant-in-Aid for Exploratory
Research from the JSPS (Number: 23651110), as well as
the previous grants such as a Grant-in-Aid for Scientific
Research for young scientist promotion from the JSPS, a
CREST grant from JST, and an Industrial Technology
Research and Development Projects grant from NEDO.
The experiments were carried out under the approval ofPhoton Factory Advisory Committee (PAC: Proposal No.

2003G301,2012G099).

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*shushi@apchem.nagoya-u.ac.jp