x-ray nanotomography of sio2-coated pt90ir10 tips with sub...
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X-ray nanotomography of SiO2-coated Pt90Ir10 tips with sub-micronconducting apexV. Rose, T. Y. Chien, J. Hiller, D. Rosenmann, and R. P. Winarski Citation: Appl. Phys. Lett. 99, 173102 (2011); doi: 10.1063/1.3655907 View online: http://dx.doi.org/10.1063/1.3655907 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v99/i17 Published by the American Institute of Physics. Related ArticlesRole of atomic terraces and steps in the electron transport properties of epitaxial graphene grown on SiC AIP Advances 2, 012115 (2012) Atomic configuration of the interface between epitaxial Gd doped HfO2high k thin films and Ge (001) substrates J. Appl. Phys. 111, 014102 (2012) Electric transport through nanometric CoFe2O4 thin films investigated by conducting atomic force microscopy J. Appl. Phys. 111, 013904 (2012) Structural variability in La0.5Sr0.5TiO3±δ thin films Appl. Phys. Lett. 99, 261907 (2011) Iron and nitrogen self-diffusion in non-magnetic iron nitrides J. Appl. Phys. 110, 123518 (2011) Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors
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X-ray nanotomography of SiO2-coated Pt90Ir10 tips with sub-micronconducting apex
V. Rose,1,a) T. Y. Chien,1 J. Hiller,2 D. Rosenmann,3 and R. P. Winarski31Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA2Electron Microscopy Center, Argonne National Laboratory, Argonne, Illinois 60439, USA3Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, USA
(Received 17 August 2011; accepted 3 October 2011; published online 24 October 2011)
Hard x-ray nanotomography provides an important three-dimensional view of insulator-coated
“smart tips” that can be utilized for modern emerging scanning probe techniques. Tips, entirely
coated by an insulating SiO2 film except at the very tip apex, are fabricated by means of electron
beam physical vapor deposition, focused ion beam milling and ion beam-stimulated oxide growth.
Although x-ray tomography studies confirm the structural integrity of the oxide film, transport
measurements suggest the presence of defect-induced states in the SiO2 film. The development of
insulator-coated tips can facilitate nanoscale analysis with electronic, chemical, and magnetic
contrast by synchrotron-based scanning probe microscopy. VC 2011 American Institute of Physics.
[doi:10.1063/1.3655907]
Materials reduced to the nanoscale often exhibit fasci-
nating properties substantially different from those displayed
by the bulk and have enormous potentials in many modern
fields such as nanoelectronics, spintronics, and energy
materials.1–3 Further progress in the area of nanoscience and
nanotechnology, however, inherently calls for the develop-
ment of advanced probes capable of achieving high spatial
resolution with concomitant chemical, electronic, and mag-
netic contrast. Scanning tunneling microscopy (STM) can
achieve the required spatial resolution. Nevertheless, direct
chemical contrast can only be obtained in very specific
cases.4 A promising candidate for next generation micros-
copy, which can meet the above requirements, is synchrotron
x-ray scanning tunneling microscopy (SXSTM).5 It com-
bines the ultimate spatial resolution of STM with the elec-
tronic, chemical, and magnetic sensitivity of synchrotron
x-rays. While the scanning probe provides the high spatial
resolution, interactions of synchrotron x-rays with matter
yield chemical, electronic, and magnetic contrast. The pros-
pects of the combination of scanning probes with synchro-
tron radiation has led to substantial efforts at synchrotron
facilities worldwide.6–13 Generally, the spatial resolution in
STM depends on the sharpness of the tip.14 However, in
SXSTM, in addition to tunneling current, x-ray photoabsorp-
tion can yield extra electrons that are ejected from the sam-
ple and collected at the tip.15 These photo-ejected electrons
are generally not only detected at the apex of the tip but also
at the sidewalls, which consequently will degrade spatial re-
solution of any measurement. Thus, “smart” tips have to be
developed and utilized. The term “smart” tip refers to probes
that are entirely coated by an insulating film except at the
very tip apex in order to minimize the background caused by
photo-ejected electrons collected through the sidewalls. So
far several techniques and coatings have been utilized for the
purpose of developing and fabricating “smart” tips.13,16–19
Our experiments were performed at the Hard X-ray
Nanoprobe (HXN) beamline, jointly operated by the
Advanced Photon Source and the Center for Nanoscale Mate-
rials, at Argonne National Laboratory. The HXN can obtain
tomographic images with 30 nm voxel resolution.20 A series
of 2D images with the sample rotated stepwise through 180�
allows reconstructing 3D tomographic data. Our datasets con-
sist of 1801 images with an acquisition time of 4 s per step.
The different attenuation power of materials in the sample
enables absorption contrast imaging. A photon energy of
10 keV was selected. Tomographic reconstruction was carried
out by the XRadia TXMReconstructor software package using
a filtered back-projection algorithm.21
Tips were electrochemically etched from a Pt90Ir10 wire
with a diameter of 250 lm using a CaCl solution. After
cleaning with alcohol and H2O, nominally 500 nm SiO2 were
deposited by electron beam physical vapor deposition
(EBPVD). In order to assure uniform coating the tips were
mounted under an angle of about 17� with respect to the
SiO2 source and rotated at 20 rpm during deposition.
A deposition rate of 0.1 nm/s was used at a base pressure of
about 6� 10�8 Torr. Figure 1 shows scanning electron
micrographs at various stages of tip preparation. After SiO2
deposition the tip is entirely coated by an insulating film.
The growth is very uniform at the sidewalls of the tip as
depicted in Fig. 1(a). The cross section was obtained by
focused ion beam (FIB) milling perpendicular to the normal
direction of the tip. A closed SiO2 film with a thickness of
about 485 nm can be observed. The dark areas in the oxide
film represent voids, which may be an artifact of the milling.
However, pinholes are not present. Due to geometry the
growth close to the tip apex is more complex. Figure 1(b)
shows the tip apex immediately after the deposition of the
oxide film. Obviously, the coating is extended to the area
above the tip. In order to dissect the apex of the tip, FIB was
utilized with Gaþ ions impinging at the apex under normal
incidence (30 kV, 50 pA). A circular write pattern was used
for the ion beam in order to sharpen the tip and “shave off”
the oxide from the apex. The result is presented in Fig. 1(c),
a)Author to whom correspondence should be addressed. Electronic mail:
0003-6951/2011/99(17)/173102/3/$30.00 VC 2011 American Institute of Physics99, 173102-1
APPLIED PHYSICS LETTERS 99, 173102 (2011)
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in which the SiO2 film is entirely removed from the apex.
Typically, this milling step leads to an uncoated tip area,
which is undesirably extended over several microns (cf., Fig.
1(d)). In order to reduce the exposed tip area and repair the
coating, subsequently an insulating SiO2 film is grown by
FIB with the ion gun located perpendicular to the tip axis
(cf., Fig. 1(e)). An ethylsilicate precursor (Si(OC2H5)4) is
applied in situ through a nozzle in close proximity to the tip.
The interactions of the precursor with the Gaþ ions cause the
deposition of an insulating film in areas were the ions are hit-
ting the tip. After multiple iterations, which include rotation
of the tip along the tip axis, an insulator coated tip with a
sub-micron exposed apex can be fabricated.
To fully elucidate the structural properties of “smart”
tips, tomographic measurements are needed. A full view of
the reconstructed insulator coated tip is presented in Fig. 2.
A video, which presents the tip rotating in space, is available
as supporting information. Qualitatively, three different
regimes can be distinguished as illustrated by the insets in
Fig. 2, which show picture details of the tip immediately at
the apex (top right), a region close to the apex, in which an
insulating film was grown by FIB (center right), and a region
that represents the primary oxide as grown by EBPVD (bot-
tom right). At the apex the Pt90Ir10 tip is exposed over a
length of only about 200 nm. Nonetheless, the procedure
described in this paper has the potential to further minimize
this exposed area by improving the FIB-assisted oxide depo-
sition. The oxide film, which was grown by FIB, exhibits a
thickness of about 100 nm (cf., orthogonal slice through the
tip in left inset). Interestingly, in some areas the oxide film is
not in direct contact with the tip surface (cf., arrow in Fig. 2,
center inset). This is most likely a consequence of the geom-
etry of the tip and the orthogonal position of the ion gun with
respect to the tip when the precursor is applied. The tip area,
which is facing the ion gun, displays a curved substrate for
the oxide growth. When the oxide grows thicker, the film
starts to engulf the curved tip and grows slightly into areas
that are averted from the ion beam source. But in these shad-
owed areas the oxide is not necessarily in contact with the
P90tIr10 tip anymore; rather, it can form a small overhanging
film. After rotation of the tip by 180� along its axis, and pro-
gressive FIB-assisted oxide growth on the previously averted
side of the tip, this overhanging part can lead to a nanoscale
cavity or fold between the Pt90Ir10 tip and the oxide film
along the tip axis. The characteristics of the FIB milling pro-
cess of the initial SiO2 film can be studied by examination
the transition area from EBPVD (cf., A in Fig. 2, bottom
inset) to FIB-assisted (cf., C in Fig. 2, bottom inset) grown
oxide. In area B some of the initial oxide film was ablated by
gallium ions grazing the tip, while it was totally removed in
area C, which had to be repaired by FIB-assisted insulator
deposition.
In order to understand the electric transport of tips under
x-ray illumination and to study the influence of the coatings,
we have exposed tips to x-rays at a photon energy of 778 eV
(Co L3 absorption edge). A photograph of a tip in a titanium
tip holder is shown in Fig. 3(a). The contour maps depicting
the tip current Itip of a bare Pt90Ir10 (Fig. 3(b)) and a “smart”
SiO2/Pt90Ir10 (Fig. 3(c)) tip are derived from scanning the tip
holder through the stationary x-ray beam of approximately
100� 100 lm2 with a flux of about 5� 1011 photons/s.
When x-rays hit the tip, absorption can lead ejection of pho-
toelectrons. The charge flow that is needed to keep the
grounded tip neutral was recorded as tip current Itip. In case
of the bare Pt90Ir10 tip Itip reaches a maximum of about 0.3
nA (cf., white oval in Fig. 3(b)). This value is reduced by
about 50% for a SiO2/Pt90Ir10 tip. In case of the “smart”
SiO2/Pt90Ir10, x-rays are impinging onto the uncoated
FIG. 1. Scanning electron micrographs of a PtIr tip for various steps of tip
preparation. After EBPVD growth, (a) the cross-section along the sidewall
and (b) the image at the apex of the tip displays a closed SiO2 film. (c), (d)
After FIB milling about 6 lm of the oxide have been removed from the tip.
(e) The exposed tip area has been minimized by in-situ FIB-assisted insula-
tor growth utilizing an ethylsilicate precursor.
FIG. 2. (Color online) The center image shows the volume rendering of the
tomographic reconstruction of an insulator-coated tip. The insets on the right
side show picture details of the PtIr core and the insulating SiO2 coating.
The left insets represent 2D orthogonal slices through the tip. A video is
available as supporting information, which shows the tip rotating in space
(enhanced online) [URL: http://dx.doi.org/10.1063/1.3655907.1].
173102-2 Rose et al. Appl. Phys. Lett. 99, 173102 (2011)
Downloaded 19 Jan 2012 to 146.137.70.71. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
Pt90Ir10 apex as well as onto the SiO2 coated area. In an
actual SXSTM experiment, the tip has to be supported by a
tip holder (cf., Fig. 3(a)), which also may collect photoe-
jected electrons from a sample or eject photoelectrons when
x-rays hit the tip holder. Figure 3(b) shows the photocurrent
of a bare titanium tip holder with a maximum of about 0.35
nA. In Fig. 3(c) a titanium tip holder was for comparison
coated with a thick (�100 lm) insulating boron nitride film,
which reduced Itip to almost zero. Generally, photo-ejected
electrons originate only from within the outer �5 nm of the
tip because of the small inelastic mean free path of electrons
in materials.22,23 All of the deeper photo-ejected electrons,
which were generated as the x-rays penetrated deeper into
the material, are either recaptured or trapped in various
excited states, most prominently in plasmon excitations. In
case of the thick boron nitride film Itip vanishes, which
means that the charge lost by ejected electrons is not refilled.
Consequently, this film undergoes strong charging effects.
This is not the case for the 100-nm thick SiO2 coating.
Because electron tunneling can be excluded for an oxide film
of this thickness, this suggests the presence of defects
induced states or deep impurities in the oxide film that can
lower the band gap.24 However, the overall tip current is sig-
nificantly reduced compared to an uncoated tip, which has
been proven to enable stable STM imaging conditions even
under x-ray illumination of the sample.
In summary, we have utilized a combination of EBCVD
and FIB in order to grow “smart” tips, which are the corner
stones for emerging tip-based microscopy techniques that
involve photon excitations. Nanotomography data show that
tips can be entirely coated by a SiO2 film leaving only a sub-
micron region at the apex exposed. Interestingly, the oxide
shell forms a nanoscale fold along the tip axis, in which the
oxide is not in direct contact with the Pt90Ir10 tip. The SiO2
coating strongly reduces the number of photo-ejected elec-
trons. The development of insulator-coated tips with ultra-
small conducting apex is indispensible for advances in
synchrotron-based scanning probe microscopy, which has
the potential to provide nanoscale imaging and spectroscopy
with chemical, electronic, and magnetic contrast.
Work at the Advanced Photon Source, the Center for
Nanoscale Materials, and the Electron Microscopy Center
was supported by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences, under contract
DE-AC02-06CH11357.
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FIG. 3. (Color online) (a) Photograph of a tip mounted on a holder used for
the photocurrent measurements. (b) The photocurrent map of an uncoated
Pt90Ir10 tip exhibits a maximum of about 0.3 nA (cf., ellipse). (c) After coat-
ing of the tip with SiO2 the photocurrent is drastically reduced. A thick bo-
ron film additionally coats the tip holder, reducing the photocurrent to
almost zero. Note that parts of the base are blocked from the incoming
beam.
173102-3 Rose et al. Appl. Phys. Lett. 99, 173102 (2011)
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