Applied Surface Science 542 (2021) 148642

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Oxidative vaporization etching for molybdenum tip formation in air
Yuto Goto a, Rie Suizu b, Yutaka Noguchi c, Toyo Kazu Yamada a, d, *a

Department of Materials Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
Department of Chemistry, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japanc
School of Science and Technology, Meiji University, Kanagawa 214-8571, Japand
Molecular Chirality Research Centre, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japanb

A R T I C L E I N F O

A B S T R A C T

Keywords:
Molybdenum
Tip probe
Oxidative vaporization
Molybdenum oxides
Scanning tunneling microscopy
Conductance

Oxidative vaporization in air using a flame with a temperature range of 1950–2300 K was employed for con­trolling the tip shape made from a nipper-cut metal molybdenum (Mo) wire edge. An extremely high vapor
pressure of MoO3 generated on the Mo surface in flame is a driving force behind the tip shape formation. Since
the MoO3 vaporization rate follows the flame temperature gradient, we could control the tip apex shape by
selecting the proper flame etching condition. The best condition to obtain a sharp tip apex based on statisticaltests of dozens of Mo tips was obtained by Mo wire edge insertion into the 2100 K flame from the side for one
second. This was repeated twice, which reproducibly provided a tip apex with a radius of 50–100 nm and a coneangle of 20–30 degrees. The present Mo tips, fabricated without aqueous solutions, were examined for theirsuitability as probe tips through air-scanning tunneling microscopy (STM), ultrahigh vacuum STM, field emission
spectroscopy, and conductance measurements.

1. Introduction
Numerous techniques for producing metallic nano tip probes havebeen widely developed in recent decades [1–19], ever since the precise
detection of electrons, holes, and ions through a single atom / moleculeor nano dot has provided a deeper insight into the charge transfer
mechanism [20,21], direct visualizations of sample surface states[22,23], molecular orbital states [24,25], and quantum spin states[26,27]. Consequently, this has offered tremendously diverse possibil­
ities for the development of atomic-size devices in single molecularelectronics [28–30], single atom / molecule spintronics [21,31–33],
topological electronics [34,35], and high-frequency devices [36,37].
Nowadays metallic tip probes support a wide variety of nanoscience
research fields, which are often used as four- or two-point probes forsurface conductance measurements [38,39] or nano-wire resistancemeasurements [40–42]. Sharp metallic tips have also been used in field
ion microscopy [43,44], field emission scanning electron microscopy(FE-SEM) [45,46], scanning tunneling microscopy (STM) [47], and qplus sensor atomic force microscopy [48].
The most commonly used tungsten (W) or noble metal platinumiridium (PtIr) probe tips have been fabricated via aqueous chemical
etching treatments [2–6,15–19,44,46]. Alongside these aqueous

chemical etching methods, a further innovative method without
aqueous solutions for constructing tip probes from metal wire have also
been developed, which employed a drastic change of the vapor pressure
between metal and its oxides. For example, tungsten has a very low
pressure of 10− 8 Torr at 2400 K (higher than the flame temperature), but
the vapor pressure drastically increases when the W changes to WO3; e.g.
760 Torr at 1900 K. Using this oxidative vaporization property with thetemperature gradient in the flame, a sharp W tip prove was produced
within a few seconds in air, by simply using a liquefied petroleum (LP)and oxygen (O2) gas flame [1].
However, this oxidative vaporization etching was not suitable fornoble metals. Since their melting temperatures are lower than a highest
flame temperature of 2300 K, the noble metal wire edge formed a ballshape instead of a tip shape. On the other hand, W and Mo have amelting temperature of 3695 K and 2890 K, respectively, which aremuch higher than the fame temperature. Therefore, they should not be
melted by the flame. Markedly, with our eyes, the W wire edge wasconfirmed to be changed to a clear tip shape via the oxidative vapor­ization. However, the Mo wire edge formed a ball shape. This is unusual
since Mo should not be melted and almost no vaporization at the flame
temperature.
Hence, the present study focuses to unveil the oxidation process at* Corresponding author at: Department of Materials Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan.
E-mail address: toyoyamada@faculty.chiba-u.jp (T.K. Yamada).
https://doi.org/10.1016/j.apsusc.2020.148642
Received 16 September 2020; Received in revised form 20 November 2020; Accepted 28 November 2020
Available online 13 December 20200169-4332/© 2020 Elsevier B.V. All rights reserved.

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the Mo wire edge inside the flame, and we will show that a sharp Mo tip
shape formation can be provided by tuning the MoO3 vaporization rate.

two regions: an inner oxygen-poor white flame and an outer oxygen-richblue flame. The flame temperature was measured using an Ir-IrRh40
thermocouple (Furuya Metal Co., Ltd.). x = 0 mm is defined as thecentre of the flame, and y = 0 mm is defined as the top of the micro torchfrom which the flame is emanating (see Fig. 1a). The flame temperature
along the flame axis (x = 0, y = 0–42 mm) denoted a highest temper­
ature of 2300 K at y = 5 mm, at the boundary between the inner and theouter flame. The temperature gradually decreased to 2100 K at y = 15mm. Subsequently, at a value greater than y = 30 mm, the temperature
dropped below 1700 K (see Suplementary Material, Fig. S1). We also
checked the flame temperature perpendicular to the flame axis (along

2. Methods
2.1. Flame etching
Fig. 1a shows the flame setup. Herein, a typical mixture of liquefied
petroleum (LP) and oxygen gas was used. We used a commercial micro
torch (an aperture with a diameter of 0.5 mm) to obtain a reproducible
flame of constant size, with the same temperature. The flame consists ofFig. 1. (a) Gas flame with x and y axes. (b) Before and after the Mo wire edge placed in the flame. (c) Optical microscopy image of the Mo wire edge after the flame
insertion. (d) Flame temperatures as a function of the x-axis. Red and blue lines denote flame temperatures measured at y = 5 and 15 mm, respectively. (e-h) SEMEDX chemical identification of the flame-etched Mo tips. (e) SEM image of apex, ball and rod regions. (f) SEM-EDX results obtained from the apex region after flame
etching (red), a Mo wire surface with polishing (blue), and a Mo wire surface without polishing (green). (g) Composite material percentages obtained from SEM-EDX
peaks obtained from the apex region after flame etching (red), a Mo wire surface with polishing (blue), and a Mo wire surface without polishing (green). (h) SEM-EDX
one-dimensional mapping along the line in the SEM image. (i) X-ray diffraction results obtained from the ball and the rod regions. X-ray diffraction intensity plot as afunction of 2θ obtained from the rod (red line) and ball (blue line) regions. Black lines denote reference plots of Mo, MoO2, MoO3 type-a, MoO3 type-b, MoO3 type-c,
and MoO3 type-d. Experimentally obtained peaks are identified as Mo and MoO2. Melted MoO2 forms the ball-shape region.
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the x-axis) as depicted in Fig. 1d, since the Mo wire was inserted into theflame from the side in this study. Along this axis, a highest temperatureof 2300 K (y = 5 mm) and 2100 K (y = 15 mm) was measured at thecenter of the flame (x = 0 mm). The temperature then decreased linearlyand dropped below 1400 K at x = 1.5 mm.
A Mo wire (diameter φ = 0.20 mm, purity 99.95%) was cut off by anipper with a length of 10–15 mm and fixed by a metal wire holder (socalled pin vise, see Fig. 1b). The wire edge was then inserted perpen­
dicular to the flame axis from the side. We used a linear course motion
stage, which ensured good reproducibility of forward and backward
insertion of the wire edge into the flame. We used a constant insertion
time period of approximately 1 s, corresponding to one etching cycle.
During these cycles, we consistently observed light flashes (see Fig. 1b,lower panel). After the flashes, the Mo wire edge always formed a ~ 0.5mm size ball. However, we found that an apex region was also formed on
the ball (see Fig. 1c).

filament) was then placed in front of the Mo tip apex (2 mm separation).
A negative bias voltage was applied to the tip side, and emission current
from the tip apex was simultaneously measured by the counter elec­
trode. An exponentially increased FE curve was obtained above thethreshold bias voltage. Our setup measured the current up to 50nA and
immediately stopped the application of the bias via a Lab View control.
2.6. Tip probe conductance measurements
A commercial vacuum probe station was used (Riko International
Ltd. i-series). The measurement setup is shown in Fig. 4a. The test
sample was an evaporated gold film prepared by the template stripping
method [52]. The thickness of the gold film was ~ 500 nm. Two probes
contacted on the gold surface, where one was a standard W probe while
the other was the flame-etched Mo probe. As a reference, a nipper-cut
Mo wire and standard W probe were also used as the second probe.
The current–voltage curves were measured in air using a source measureunit (Keithley 2636A).

2.2. SEM, EDX and X-ray diffraction measurements
A compact Tiny-SEM (Technex Lab. Co. Ltd., operation pressure 10− 2Pa, electron beam energy 10 keV) was used to verify the shape of the Mo
wire edge before and after the flame etching. SEM images were obtained
within 30 min of the flame etching. Chemical identification of the flame
etched Mo tips was performed with X-ray diffraction (XRD) and SEM
energy dispersive X-ray (EDX) spectroscopy. X-ray data were recordedwith the Bruker AXS Smart APEX Ultra by using graphitemonochlomated CuKα radiation (λ = 1.54178 Å). The SEM JSM6510A (JEOL) was used for EDX measurements.

3. Results and discussion
Fig. 1e shows a SEM image of the flame etched Mo tip in the vicinityof the apex. We divided this area into three distinct regions: apex, ball,and rod. The apex region has a smooth surface, and grains or grainboundaries are not observed. The ball region includes many grains. The
rod region has a smooth surface and no grain formation is observed.
SEM-EDX measurements were performed for the apex, ball, and rod
regions by focusing the electron beam on each region. All regions
showed peaks at the same energy levels, and it was found that there were
three chemicals on the surface, namely Mo, oxygen (O), and carbon (C)(see Fig. 1f). The presence of the carbon peaks in most likely explainedby the fact that we used a carbon tape as a background substrate. EDX
quantitative analysis results are summarized in the table in Fig. 1g. We
found that the apex surface of the flame-etched Mo tip consists of Mo:
81% and O: 19%. Meanwhile, one might note that the EDX results
indicate oxidization at the apex. This oxidization could be occurred
during the LP + O2 flame etching or natural oxidization in air after theflame treatment. In order to understand how the flame etching con­tributes to the oxidization, we compared EDX measurements for the Mo
apex before and after the flame etching. The results were shown in thetable in Fig. 1g. Before the flame etching, the Mo apex surface consists of
Mo: 39% and O: 61%, namely the Mo surface in air is coated by a thickoxide film. However, after the flame etching, a drastic decrease of theoxidization occurred: Mo: 81% and O: 19%. As a reference, we also
polished the Mo wire surface and removed the thick oxide film. Then thepolished Mo showed Mo: 80% and O: 20%, which is comparable to theflame-etched Mo tip apex. These prove that the flame etching does notenhance the oxidization, but excludes the thick oxide film. The remained
oxygen peak in the EDX measurement could be due to oxidization in air
after the flame etching. One-dimensional EDX mapping was also per­formed on the Mo tip apex region (see Fig. 1h), which clearly revealed
that the Mo is the majority element at the apex.
Further chemical identification of the flame etched Mo tip was per­formed using X-ray diffraction (XRD). Fig. 1i shows XRD data obtainedat the ball and rod regions. By comparing this data with the reference
data of Mo metal and Mo-oxides (MoO2 and four polymorphs of MoO3,
see Supplementary Material, Fig. S5), the experimentally obtained XRDpeaks at the ball and rod can be identified as MoO2 and Mo, respectively.
We further obtained Laue spots at the apex, ball, and rod regions (seeSupplementary Material, Fig. S4), indicating that neither the apex nor
the ball regions consist of a single domain, but rather each includes
several larger differently oriented domains. However, the rod region did
not exhibit any orientation (polycrystal).
The XRD and SEM-EDX results in Fig. 1e-i identify that the generatedball consists of MoO2, while the tip apex consists of Mo. No MoO3 wasremained. These experimental evidences are well explained by the Mo-

2.3. UHV-STM measurements
A home-built ultrahigh vacuum (UHV) STM setup operated at 300 K
was used. The setup included introduction, preparation, and analyticalchambers (base pressure: 10− 8 Pa). The gate valves separated eachchamber; therefore, we could transfer the tip and an Au(0 0 1) sample
through the introduction chamber without breaking the vacuum of othertwo chambers. The tip was flushed to 2300 K (emission current 60–80
mA with acceleration bias 0.5 kV) inside the preparation chamber toreduce the Mo-oxide film thickness [44]. A clean and atomically-flat Au(0 0 1) surface was obtained by repeating Ar+ sputtering (750 eV) andannealing (743 K) cycles in the preparation chamber. The cleaned tip
and sample were transferred to the analytical chamber and set into the
STM. STM and scanning tunneling spectroscopy (STS) measurements
were carried out by combining the Nanonis BP4 SPM controller and thecorresponding software. The topographic images were obtained in aconstant current mode. STS was carried out by measuring tunnelingcurrent as a function of the sample bias voltage (I(V) curve) at each pixel
position in an STM topographic image (feedback off). Backgroundexcluding treatments of the original STM topographic images, and theprocess of extracting the I(V) curves from the original data, were per­formed with the WSxM 5.0 Develop 9.1 software [62].
2.4. Air-STM measurements
A commercial STM setup was used in air (JEOL 4200) [63,64].
Highly oriented pyrolytic graphite (HOPG) was cleaved by using a
scotch tape in air and then set inside the STM stage. The flame-etched
Mo tip was approached on the cleaved surface and STM measurements
were performed in a constant current mode using the JEOL software. I(V) curves were measured under feedback off. The WSxM 5.0 Develop
9.1 software was used for data analysis [62].
2.5. Field emission curve measurements
A home-built UHV field emission (FE) setup was used [44]. The
flame-etched Mo tip was set in the tip stage. A counter electrode (W3

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oxide phase transition process [49–51,53,54]. Mo first becomes MoO3 at
673 K, and then changes to MoO2 above 700 K. Interestingly, MoO2 has alower vapor pressure (10− 4 atmosphere (atm) at 1818 K) with a melting
point of 1373 K, which is the cause of the MoO2 ball generation at thelower flame temperature region. On the other hand, MoO3 has extremely
high vapor pressures: e.g. 10− 3 atm at 1000 K, 0.5 atm at 1350 K, and 1
atm at 1424 K, namely the generated MoO3 inside the flame wasimmediately vaporized, and therefore there is no MoO3 after the Mo
flame etching as confirmed by the XRD measurements in Fig. 1i. Becauseof the linear temperature gradient inside the flame along the x axis (seeFig. 1d), the center of the flame has the highest MoO3 vaporization rate,
thus providing the tip shape.
Now we focus on the fabrication process of the apex region. Fig. 2ashow SEM images obtained from the Mo wire edges after the flame
insertion using different temperatures: from left to right, 1450, 1850,
1950, 2100, 2250 and 2300 K. Although MoO3 was generated above
673 K and the vapor pressure increases to 1 atm at 1424 K according to
the phase diagram [49–51,53,54], the MoO3 vaporization rate is notenough to produce a tip shape for 1450–1850 K. We could still see theoriginal nipper-cut edge shape. However, starting from 1950 K to 2300
K, the apex showed a clear tip shape. It is noted that the cone angle of theapex changed at different temperatures. The 2100–2250 K flame pro­
duced a sharper cone angle, but the 2300 K flame produced a blunter
apex, indicating too high MoO3 vaporization rate could render the apex
blunter.
Fig. 2b shows models of generation and vaporization of MoO3 inside
the flame. As shown in Fig. 2a, depending on different temperature
flames, regular to irregular shape morphology was produced at the apex.
Because of the insertion position into the flame, we could choose
different temperature gradient inside the flame (see Fig. 1d), and thistemperature gradient controls the apex morphology. Three models in
Fig. 2b represent how different temperature gradients change themorphology using larger MoO3 vaporization rate at higher temperature.
Color bars indicate the flame temperature. The size of the black circles
denotes a vaporization rate of MoO3 at each position, which is deter­
mined by local temperature. Using the flame with a temperature range

Fig. 2. (a) SEM images of Mo tip apexes flame-etched at 2300 K, 2250 K, 2100 K, 1950 K, 1850 K, and 1450 K. (b) Models of MoO3 vaporization etching at different
flame temperatures. Left panel: 1400–1900 K, center panel: 1400–2250 K, and right panel: 1400–2300 K. (c) Statistical data obtained from hundreds of SEM images
for dozens of flame-etched Mo tips, representing a dependence of L, φ, θ, and r on the flame-etching cycles. Mo tips etched at y = 5 mm (2300 K, red dots) and y = 15mm (2100 K, blue dots) were used. (d) SEM image of the flame-etched Mo tip. Apex length (L), ball diameter (φ), cone angle (θ), and tip radius (r) are defined. (e)SEM images of the Mo tip apexes after the flame etching cycles of 1, 2, and 8 times.
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of 1400–1900 K (left model), the MoO3 vaporization rate is low and no
big difference between the rod and the apex regions, hence the Mo wire
becomes narrow, but no tip shape was formed.
However, when we use the flame with a temperature range of1400–2250 K (see center model in Fig. 2b), the MoO3 vaporization rate
at the apex is suitably higher than the rod region, a tip-shapemorphology is formed, while if one uses the 1400–2300 K flame, thevaporization rate at the apex region becomes too high, hence rendering
the tip apex blunter (see right model in Fig. 2b).
During the flame etching, we found that the Mo atoms in the apex
region moved even the flame temperature was much lower than themelting temperature of 2890 K since X-ray Laue spots appeared after the

Fig. 3. Testing the flame-etched Mo tip probes. (a)Field emission (FE) spectroscopy measurements. FE
curves were obtained after 5 (blue dot) and 11 (reddots) flushing treatment in UHV. Fitted FowlerNordheim plots show the tip radii of 110 and 53
nm. (b) Scanning tunneling microscopy (STM)
model. (c) STM topographic image obtained on acleaved HOPG(0001) surface in air (1890 × 910
nm2, Vs = − 1 V, It = 500 pA). The right panel de­notes an I(V) curve obtained on the HOPG(0001)(setpoint Vs = − 1 V, It = 1 nA). EF denotes the Fermi
energy. (d) STM topographic image obtained on acleaned Au(0 0 1) surface at 300 K in UHV (50 × 30
nm2, Vs = − 1 V, It = 1 nA), and the enlarged image(right panel, 9.5 × 4.8 nm2, Vs = − 100 mV, It = 1nA). (e) Height profile along the arrow in (d). (f) I(V) and dI/dV curve obtained on the Au(0 0 1) (set­point Vs = − 1 V, It = 1 nA). EF denotes the Fermi
energy.

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flame etching (see Supplementary Information). When MoO3 wasformed and immediately vaporized, fresh Mo atoms appeared. These Mo
atoms were immediately changed to MoO3 because of the presence ofoxygen in the flame, and vaporized again. This process was repeated.
Simultaneously, the Mo atoms could diffuse on the surface and form aflat and smooth surface at the apex.
Thus, proper MoO3 vaporization etching rate is required to obtain asharp tip formation. We systematically flame-etched and carefully ana­
lysed the tip apexes using SEM to find the best conditions to provide a
sharper tip shape formation with a good reproducibility. Fig. 2c shows
the statistical results obtained based on dozens of the flame etched Mo
tips from hundreds of SEM images (some examples are shown in Sup­
plementary Material, Figs. S2 and S3).
To perform the statistical measurements, it was necessary to define
the flame etching process in further detail. We used the course motion to
reproduce the same flame etching condition (Fig. 1b). We placed the Mo
wire edge into the flame from the side, and waited 1 s during the
flashing, then pulled the Mo wire from the flame. This 1 s flashingmotion was defined in this study as one etching cycle. We repeated thisprocess 1–10 times. At each cycle, the tip shape was monitored using
SEM. Based on the hundreds of the SEM images obtained, we quanti­tatively characterized four parameters as depicted in Fig. 2d: a length of
the tip formation region (L), a diameter of the ball region (φ), a coneangle of the apex (θ), and a tip radius (r). The tip radius r is the radius of
the local curvature at the forefront tip apex, which was obtained by aparabolic curve fit: y = ax2 + bx + c and r = (2a)− 1. Fig. 2c shows asummary of the data obtained at a flame position of y = 5 mm (red dots,
2300 K) and y = 15 mm (blue dots, 2100 K).
The length of the tip apex region (L) varied between 100 and 600 μm(see Fig. 2c), but it seems that etching with a 2100 K flame produced alarger tip apex length. This is a good agreement with the models in
Fig. 2b, namely the 2300 K etching was too powerful and made the apex
length shorter. This also relates to the cone angle (θ) and the tip radius(r), i.e., the 2100 K etching provides sharper tips. Further, the MoO2 balldiameter was approximately 400 μm for 2100 K and approximately 600
μm for 2300 K, indicating the higher temperature encouraged more Mo
changes to MoO2. Repetition of the flame etching did not produce sig­
nificant changes of the ball size.
Fig. 2e shows SEM images obtained from the Mo tip apex after 1, 2
and 8 cycles of the 2100 K etching. The cone angle is drastically changedby the numbers of the cycles. Therefore, the number of etching cycles isalso important factor to get a sharp apex.
Through these statistical measurements in Fig. 2c, the 2100 K flame
etching was better to produce a sharper tip rather a highest temperatureof 2300 K. So far we found that the best condition to reproducibly
produce an apex with a cone angle of θ = 20–30 degrees and a radius of r
= 50–100 nm was a Mo wire edge insertion into the 2100 K flame (y =15 mm) for 1 s repeating twice (see Fig. 2c, marked by a dotted circle).
Chemically etched Mo tip probes have been used as a scanning probe
tip, a field emission cathode tip, and a sample holder in nano-scientificresearch [55–57]. Here, we show that the oxidative vaporized Mo tips
also provided the same quality. Firstly, a field emission test in UHV wascarried out (see Fig. 3a) [44]. The flame etched Mo tip in air was
introduced into the UHV field emission (FE) tip stage; subsequently,
negative bias was applied to the Mo tip (0–3000 V, producing a 0–106 V/
m electric field). No field emission was produced, suggesting that theflame-etched Mo tip apex could be covered by an oxide film or becontaminated. This is normal since the flame etching process was done
in air. However, after five flushing cycles in UHV (2300 K) using electron
bombardment, we successfully measured a field emission current shown
as red dots in Fig. 3a. Since this is similar to the chemically etched W tip
case [44], the flame-etched Mo tip apex could be coated by an oxide thinfilm due to oxygen absorption in air as pointed by the SEM-EDX mea­
surements in Fig. 1g. The radii of the flame-etched Mo tips were ob­tained with a fit of Fowler-Nordheim plot based on a semi-spherical apex
model (solid lines in Fig. 3a). The experimentally obtained fieldemission (I) curves as a function of the applied bias (V) to the tip was
fitted by I = (2πr2) × 1.537 F2 × 1014/ (Φt2(y)) exp (− 0.683 Φ1.5 ω(y) /F), where F denotes the electric field at the apex: F = 0.368 V / (r0.9 (d0.1–0.75 r0.1)), y = 3.79 F1/2 / Φ, t(y) = 0.9967 + 0.0716 y + 0.0444y2,
ω(y) = 1.0029 – 0.1177y + 1.1396 y2 – 0.2561 y3, tip-anode distance (d): 0.002 m, work function of Mo(1 1 0) (Φ) : 4.95 eV [8,44]. The tip radius
was evaluated as r = 110 nm. We further repeated the UHV flushing. The
blue dots were obtained after 11 flushing cycles with a radius of r = 53
nm, indicating a further reduction of the Mo-oxide film thickness. The
range of radii for flame-etched Mo tips was thus shown to be 50–100 nm.
Secondly, the flame-etched Mo tip was tested as an air-STM probe tip(see Fig. 3b). A flame etched Mo tip was set in the air-STM setup andapproached on the cleaved HOPG surface. Although we expected a crashof the flame-etched Mo tip because of the oxide film coating on the Mo
tip apex, the Mo tip detected the tunneling current and safely stopped
the approach without a tip crash in air. The obtained STM topographic
image is shown in Fig. 3c. Atomic terraces and monolayer steps areobserved, but scratch-like noises are also observed. The I(V) curve ob­tained on the same surface showed no band gap around the Fermi energy
and displayed an exponential increase for both the negative and positive
bias sides, indicating the metallic property of the flame etched Mo tip in
air. One might deduce that the scratch-like noises included in the
topographic image and the I(V) curve could be due to the remaining Mooxide coating the apex.
Thirdly, the flame etched Mo tip produced in air was tested as a UHVSTM probe tip [58–61]. Here, the Mo tip was flushed to clean the apex in
the UHV preparation chamber. Fig. 3d shows an STM topographic image
obtained on a clean Au(0 0 1) surface with the flame etched Mo tip.
Surface reconstructed (5 × 20) or (5 × 28) stripe patterns along the〈1 1 0〉 direction are clearly observed on the atomic terraces [65]. A
height profile along the blue arrow in Fig. 3d is shown in Fig. 3e, where a
fcc-Au(0 0 1) monolayer step height of ~200 pm is confirmed. An
enlarged STM image obtained on the terrace emphasized the surface
reconstruction. The height profile along the black arrow indicates a
stripe with a 1.3 nm periodicity (20 pm height corrugation). Spheremodels denote top and side views of the Au(0 0 1) surface reconstruction,
where red and white ball correspond to surface and sub-surface atoms,
respectively. I(V) and dI/dV curves obtained on the Au(0 0 1) surface
showed no gap (dI/dV ∕= 0 at the Fermi energy), but did show an
exponential increase for both the negative and positive bias sides,
indicating that the Mo tip has metallic properties (see Fig. 3f).
Comparing the results in Fig. 3f with the ones in Fig. 3c, the scratch-likenoise decreased drastically, indicating that UHV flushing is a promisingtreatment for reducing the Mo-oxide adsorbed on the apex. Since these
testing results showed a comparable quality with the STM results ob­
tained by using a chemically etched Mo tip [55], the flame-etched Mo tip
could be also used as an STM probe tip.
Finally, we examined the flame-etched Mo tips as a probe for a twopoint contact conductance measurement. Fig. 4a show the setup. One
electrode was used as a standard W tip probe. As the other electrode, we
examined several tip probes, namely two flame-etched Mo tips (No.1
and No.2), a commercial W tip, and a nipper-cut Mo wire. Fig. 4b shows
the obtained I-V curves. All represents the Ohmic conductance. Since thenipper-cut Mo wire was coated by the oxide film (see Fig. 1g), itsconductance was quite low (R = 79 Ω, dotted line). However, the flameetched Mo tips showed almost 20 times larger conductance (R = 4.2 and4.3 Ω, red and black solid lines), which are comparable to the com­
mercial W tip probe (R = 3.7 Ω, blue solid line).
Experimental evidence outlined in Figs. 3 and 4 showed that theapproximately 1 s flame etching at 2100 K performed twice, led to the
production of a Mo tip shape with a radius of 50–100 nm; these Mo tips
were confirmed to be available as probe tips through STM, FE, andconductance measurements. Since the flame etched Mo tip is prepared inair, any resultant contamination from the air is best excluded by UHV
flushing, which also produces less noise data. Through the test mea­
surements in Figs. 3 and 4, we did not see any effects from the MoO2 ball,6

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Fig. 4. Testing the flame-etched Mo tip probes.
(a) Two-point contact conductance measure­ments. We used a standard W tip probe as the lefthand side probe and a Au film was used as a test
sample. As the other tip probe, we used two
flame-etched Mo tips (No.1 and No.2), a com­
mercial W tip, and a nipper-cut Mo wire. (b)Conductance measurement results obtained fromtwo flame-etched Mo tips: No.1 (black solid line)
and No.2 (red solid line), a commercial W tip(blue solid line), and a nipper-cut Mo wire (dottedline).

which is located 0.2–0.5 mm far from the tip apex.

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4. Conclusions
In summary, oxidative vaporization etching process in the LP and O2
mixture gas flame with a temperature range of 1950–2300 K was
employed to change a Mo wire edge to a tip shape in air. Especially, justby inserting a nipper-cut Mo wire edge within one second into the flamewith a temperature of 2100 K (15 mm above the micro torch top) in air
and repeated this motion twice, a sharp tip apex was reproducibly
fabricated. The flame-etched Mo tips without any treatments in vacuum
showed no crash of the tip and provided normal STM, field emission, andconductance results. Although a MoO2 ball was always generated, we
found that the Mo tip apex region was also simultaneously produced,
which was occurred by the generation and immediate vaporization of
MoO3. No MoO3 remained at the apex when we pulled out the Mo wirefrom the flame.
CRediT authorship contribution statement
Yuto Goto: Conceptualization, Methodology, Validation, Investiga­
tion, Writing - original draft. Rie Suizu: Validation, Investigation.
Yutaka Noguchi: Investigation. Toyo Kazu Yamada: Conceptualiza­
tion, Methodology, Validation, Investigation, Writing - review & editing,
Supervision, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
This work was supported by JSPS KAKENHI Grant Numbers
19H05789 and the Murata Science Foundation. We thank Mr. Satoru
Sasaki for assistance of field emission spectroscopy measurements.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.apsusc.2020.148642.
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