Fabrication of tungsten tip probes within 3s by using flame etching
Cite as: Rev. Sci. Instrum. 90, 063701 (2019); https://doi.org/10.1063/1.5085251
Submitted: 11 December 2018 . Accepted: 21 May 2019 . Published Online: 12 June 2019
Takayuki Yamaguchi, Eiichi Inami, Yuto Goto, Yuta Sakai, Satoru Sasaki, Teruaki Ohno

, and Toyo

Kazu Yamada

Rev. Sci. Instrum. 90, 063701 (2019); https://doi.org/10.1063/1.5085251© 2019 Author(s).

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Fabrication of tungsten tip probes within 3 s
by using flame etching
Cite as: Rev. Sci. Instrum. 90, 063701 (2019); doi: 10.1063/1.5085251
Submitted: 11 December 2018 • Accepted: 21 May 2019 •Published Online: 12 June 2019
Takayuki Yamaguchi,1 Eiichi Inami,1,2
and Toyo Kazu Yamada1,4,a)Yuto Goto,1 Yuta Sakai,1 Satoru Sasaki,1 Teruaki Ohno,3AFFILIATIONS
1

Department of Materials Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

2

School of Systems Engineering, Kochi University of Technology, 185 Miyanokuchi, Tosayamada, Kami, Kochi 782-8502, Japan

3

Technex Lab Co. Ltd., 8-4-4, Naruse Machida-shi, Tokyo 194-0044, Japan

4

Molecular Chirality Research Center, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japana)E-mail: toyoyamada@faculty.chiba-u.jp

ABSTRACT

A tungsten (W) tip has been used as a standard tip probe because of its robustness at the highest boiling temperature; the use cases include
a field emission (FE) electron source for scanning electron microscopy (SEM) and a scanning probe microscopy tip. The W tip probe has
generally been fabricated through a chemical etching process with aqueous solutions. In this study, we propose a new method—flame etching.
Without using aqueous solutions, a W tip probe was successfully fabricated within 3 s in air, which is very fast and convenient, and beneficial
for mass production (additionally, no expensive setup is necessary). A W tip probe was obtained simply by putting a W wire into an oxygenliquefied petroleum (O2 +LP) gas flame (giving the highest temperature of ∼2300 K) through a microtorch for a few seconds. The obtained W
tip provided atomically resolved scanning tunneling microscopic images. Also, since FE electrons were detected by applying ∼106 V/m, thetip can be used as an FE-SEM source. Generation and vaporization of WO3 on the W surface are important processes to form a tip shape.
Published under license by AIP Publishing. https://doi.org/10.1063/1.5085251

INTRODUCTION
Tungsten (W) has been used to fabricate a microscopy tip
owing to its robustness and extremely high melting point (3695 K).
Indeed, the first atomically resolved images were observed using a W
tip apex by means of field ion microscopy (FIM) in the 1960s.1 The
W tip has been used as a field-emission emitter of scanning electron
microscopy (FE-SEM)2 and also as one of the standard probes forscanning tunneling microscopy (STM) or atomic force microscopy(AFM) q-plus sensor, which visualize atomic structures of material surfaces and have aided in the development of surface science
research.3–5 Moreover, such a robust W tip was utilized to fabricate
a molecule-functionalized tip by picking up a single molecule on topof the W tip, rendering an extremely high-resolution atomic image
inside a single organic molecule.6–8 Furthermore, immiscibility of W
with other magnetic metals has enabled us to fabricate a magneticfilm coated W tip, which is widely used to perform spin-polarized
STM.9–13

Rev. Sci. Instrum. 90, 063701 (2019); doi: 10.1063/1.5085251
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The most common method to fabricate this W tip uses chemical etching with aqueous KOH or NaOH solutions (2–5 N).14–18
This wet etching process was established, but still requires significant knowledge to exclude thick (20–30 nm) oxide films andunknown impurities on the W apex adsorbed from the aqueous
solution. Indeed, such adsorbed chemical impurities result in instability of field emission and scanning imaging.19 The simplest method
is rinsing the apex with acetone or hot water, but researchers
have developed extra treatments: self-sputtering,20,21 field evaporation,22,23 field-assisted etching,24,25 ion milling,26 and controllable
crashing,27 in ultrahigh vacuum (UHV) conditions.
In this study, we propose a new method—flame etching. Without any liquid solutions or chemical materials, a W tip probe was
successfully fabricated within 3 s in air, which is very fast and convenient, and beneficial for mass production (additionally, no expensive
setup is necessary). By simply putting a W wire into a gas-burner
flame (∼2300 K) through a microtorch within 3 s, a W tip-shapeprobe was obtained. Because it is known that the wire apex formed

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the air condition is an important factor in forming the tip shape.
Generation and vaporization of WO3 were found to be important processes to produce the tip shape. A bare W tip apex was
obtained inside the flame due to the extremely high WO3 vapor
pressure around the tip apex. The obtained W tip provided atomically resolved STM images. Also, since field emission electrons weredetected by applying ∼106 V/m, the tip can be used as an FE-SEM
source.
EXPERIMENTAL
A home-built flame etching setup for metal wires is shown in
Fig. 1(a). This setup was used only to investigate the flame etchingprocess step by step. We found that such a setup is not necessary,
but a W tip was obtained only by putting the wire into the flame
for a short period by hand (see supplementary material, movie).
The setup consists of a metal wire holder (so-called pin vise) anda microtorch where the flame came out through an aperture witha diameter of 0.5 mm along the y-direction. A pin vise holding a
metal wire was attached to the x-direction course motion to control smooth linear motion (±10 mm within ∼0.5 s). Mixed oxygen(O2 ) and liquefied petroleum (LP) gas were used for the burner flame

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fuel gas. Figure 1(b) shows the burner flame (O2 +LP) in our experiment. Depending on the oxygen concentration, the temperature
within the flame changes with the distance from the microtorch top
L. In our setup, the inner flame (oxygen-poor white flame) region isL ≤ 5 mm, while the outer flame (oxygen-rich blue flame) regionis L = 5–43 mm.28 The flame temperature was measured using an
Ir–IrRh40 thermocouple (Furuya Metal Co., Ltd.) [see Fig. 1(b)].
The highest temperature of ∼2300 K was measured at L ∼ 5 mm.
The flame temperature linearly decreased from 2300 K (L ∼ 5 mm)
to ∼1500 K (L ∼ 40 mm).
Flame etching was performed using metal wires (diameter𝜑 = 0.1–0.3 mm, purity 99.95%). These metal wires were cut off
by a nipper with a length of 15 ± 5 mm and then fixed to the pin
vise. Using the linear motion controller, the cut edge of the metal
wire was inserted into the flame from the x-direction (perpendicular
to the flame direction) [see Fig. 1(c)]. Safety glasses were required.
With the setup shown in Fig. 1(a), the period that the wire apex was
exposed in the flame was controlled at ∼0.5 s, while the period canbe decreased down to ∼0.1 s by shaking one hand [without using the
setup in Fig. 1(a)].
Chemical identification of the adsorbed species on the flamed
W surfaces was performed using X-ray diffraction (XRD) andSEM energy dispersive X-ray (EDX) spectroscopy. XRD data were

FIG. 1. (a) Our flame etching setup consists of flame (LP gas mixed with O2 gas)through the microtouch. (b) An enlarged
image of the flame. White inner flame (L= 0–5 mm) and blue outer flame can beobserved (L = 5–43 mm). Temperatureinside the flame was measured by usinga Ir–IrRh40 thermocouple. (c) Enlarged
image of (a) during the flame etching. (d)SEM images of metal wire edges with different melting temperatures after baking
by the flame for 0.5–2.5 s (magnification
×30).

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recorded with a Bruker AXS D8 Advance spectrometer (Cu tube,
40 kV). SEM JSM-6510A (JEOL) setup was used to perform EDX
measurements. The apex of the wire was characterized by Tiny-SEM(Technex Lab. Co. Ltd.) under pressure of approximately 10−2 Pa.
The energy of the electron beam was 10 kV.
A home-built ultrahigh vacuum STM setup operated at 300 K
was used to test the flame-etched W tip performance. The STM
was controlled with Nanonis SPM control system. Obtained STM
images were analyzed with WSxM 5.0 Develop 9.0 software (Nanotech Electronica SL). The setup consists of three UHV chambers: introduction (base pressure: 5 × 10−8 Pa), preparation (basepressure: 1 × 10−8 Pa), and analytical chambers (base pressure:<1 × 10−8 Pa). These chambers were connected by the UHV gate
valves. Therefore, the tip was transferred between the chambers
without breaking vacuum. The STM was located in the analytical chamber. The flamed-etched W tip prepared in air was introduced from the introduction chamber. The chamber was pumped
from atmosphere to ∼10−6 Pa within 1 h. Then, we opened the gate
valve and set the tip into the STM. A Au(110) single crystal surface was used. The fcc-Au(110) surface forms missing row surface
reconstruction with a p(1 × 2) periodicity. The Au(110) surface wascleaned in the preparation chamber by repeating several cycles of
Ar+ sputtering (0.75 keV) and annealing (743 K). We also tested thetip performance on the Au(110) using the commercial STM setup(JEOL-4200) in air. The flame-etched W tips detected tunnelingcurrent without crashing even in air (see supplementary material,
Fig. S4). The STM images were obtained by using a constant current mode (feedback on), while the tunneling current as a functionof the sample bias voltage [I(V) curve] was measured by keeping thetip-sample separation (feedback off).
RESULTS AND DISCUSSION
Flame etching of various metal wires
First, we placed various metal wire edges into the flame for
0.5–2.5 s at L ∼ 5 mm (∼2300 K). Figure 1(d) shows the obtainededges after baking. Because Au, Ag, Co, Fe, and Pt have lower melting temperatures in bulk, the edges formed a sphere shape due tolocal melting. Interestingly, Mo and Ta wires also formed a sphereball shape while their melting temperature in bulk is higher than the
flame temperature of 2300 K. However, markedly, W wire with thehighest melting temperature of 3695 K in bulk formed a tip shape.
This unexpected result was the starting point of this flame etching
study.
Flame etching for W wires
Figure 2 shows a systematic study of the flame etching of the W
wires using the setup in Fig. 1. We studied how the shape of the W
wire apex changes with an increase in the total flame etching time t(O2 +LP gas, L = 5 mm). Strong light flashed during the flame etching. Indeed, this flashing is an important signature for successful
flame etching. Without strong flashing light, no etching occurred.
We started to count time when the wire apex was placed at the center of the flame (t = 0 s) [see Fig. 1(c)]. Figures 2(a)–2(e) show SEM
images of one W wire. Before the flame etching, the W wire exhibited
sharply cut edge [see Fig. 2(a)]. However, only 0.5 s of flame etching rendered the edge rounder [Fig. 2(b)]. We continued the etching

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and checked the edge shape every 0.5 s. Drastic change occurred at
t = 1.5 s [Fig. 2(c)]. Here, the wire body shrank to ∼50%. Additionally, the apex started narrowing. Markedly, at t = 2.5 s [Fig. 2(d)],the apex changed to a tip shape. Once the tip shape was formed,
further baking did not significantly change the shape [see Fig. 2(e),t = 7.0 s]. We repeated the flame etching for 9 W wires and checked
the apex shape every 0.5 s with, in total, 99 SEM images (seesupplementary material, Fig. S1). Markedly, all W wires formed atip shape, promising good reproducibility. These consecutive results
demonstrate that the present flame etching method is capable offabricating a W tip within a few seconds.
To obtain deeper insights into the tip formation, we have statistically analyzed the obtained SEM images. In this analysis, wequantitatively characterize the wire shape by three parameters: thebody diameter of the wire w, cone angle of the apex θ, and the tip
radius r. The body diameter w is defined as the distance between
solid lines in Fig. 2(f) at the foremost tip apex. The tip radius r is
the radius of local curvature at the foremost tip apex [see Fig. 2(h)],which was obtained by parabolic curve fitting: y = ax2 + bx + c andr = (2a)−1 .29 Figure 2(g) shows the obtained w, θ, and r values as afunction of t. More and more flame etching decreased the w value
starting from 300 µm to ∼50 µm within 3 s. However, additional
etching did not change the w values. Angle θ was able to measurefrom t = 2.5 s when the tip shape was formed. Starting from 40○ ,it increased to approximately 60○ by further flame etching. The tip
radius r values decreased below ∼10 µm within t ∼ 2 s. Using the
setup in Fig. 1, the radius was typically 0.5–5 µm by further flame
etching.
From this statistical experimental analysis, W wire apexes withnarrow body (w < 100 µm), small cone angle (∼40○ ), and small tip
radius (r ∼ 1 µm) were obtained around t ∼ 2.5–3.0 s with a higher
probability. However, using the setup in Fig. 1, it was difficult toobtain the apex with a radius lower than 1 µm. We expected thata sharper tip could be fabricated between t ∼ 2.5 and 3.0 s. Then,we further performed the flame etching by shaking one hand as
fast as possible. This swift flame etching gave a short flame etching period of ∼0.1 s. First, we placed the W wire into the flamefor t ∼ 2.5 s and obtained the tip shape with a narrow body. Next,
we inserted the apex, using one hand, into the flame every ∼0.1 s(L ∼ 5 ± 1 mm) for 1–3 times. The W tip apexes before and after the∼0.1 s flame etching are shown in Fig. 3(a). The blunter apex was
drastically sharpened and several grains can be observed, i.e., theapex has a polygonal shape. Because a similar shape was observedat the tip apex fabricated from the single crystalline W wire,19 theflame etched W apex could include several single crystal grains. To
measure the local curvature of such an apex, we performed fieldemission curve measurements in a vacuum using the setup in Ref. 19.
Figure 3(b) shows the obtained results. Because the W tip surface
was coated by thin oxide layers, a gentle annealing of 1500–2300 K(30–40 W) was performed. The W tip radius is known to not be
melted by this annealing.19 We observed the onset of the field emission currents around 1000–2000 eV. From the Fowler-Nordheim(F-N) fitting, we found the tip radius of r ∼ 20–52 nm at t ∼ 2.7 s[open circles in Fig. 2(g)]. The differences between the F-N fitting curves and the experimentally obtained data could be becauseof using the semispherical apex model for the F-N plot, while theobtained W tip apex had polygonal grains [see Fig. 3(a)]. Since
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FIG. 2. (a)–(e) SEM images of the W
wire after the flame etching. Total flame
etching time is (a) 0 s, (b) 0.5 s, (c) 1.5s, (d) 2.5 s, and (e) 7.0 s. (f) The body
diameter of W wire (w) was obtained by
setting two straight lines along the bodyof the wire and measured the distance
between the lines at the apex position.
(g) Dependence of the body diameter(w), the cone angle (θ), and the tip radius(r) of the W wires on the total flame
etching time (t). Sharper tips were fabricated with swift flame etching by shaking one hand (white circles). Radii of theobtained W tips were evaluated from thefield emission curves in Fig. 3. (h) The
tip radii in (g) (black dots) were simply obtained by fitting the parabolic function to the tip apex in the SEM image:ax 2 + bx + c, r ∼ (2a)−1 .

electric field: ∼106 V/m (= 1 kV/1 mm), the obtained W tip canbe used as a field emission scanning electron microscopy (FE-SEM)
source.
Further, the obtained sharper W tip was tested as an STM probe[see Figs. 3(c)–3(e)]. The flame-etched W tip was immediately set
to the UHV-STM setup at 300 K. Figure 3(c) shows an STM topographic image obtained on the Au(110) surface in UHV at 300 K.
Several atomic terraces are observed. The line profile along the arrow(left to right) in Fig. 3(c) showed ∼150 pm height steps [see the lower
panel in Fig. 3(c)], which is in agreement with the bulk fcc Au(110)plane distance of 144 pm. One dimensional lines along ⟨110⟩
direction show the fcc-Au(110) p(1 × 2) missing row pattern. The
enlarged image [Fig. 3(d)] shows the atomic structure on the reconstructed surface. The line profile along the arrow (top to bottom)
in Fig. 3(d) shows periodic peaks with an interval of ∼0.8 nm,
corresponding to the p(1 × 2) structure of 408 × 2 = 816 pm.
Tunneling current as a function of sample bias voltage [I(V)curve] was measured on the Au surface [see Fig. 3(e)]. The obtained

Rev. Sci. Instrum. 90, 063701 (2019); doi: 10.1063/1.5085251
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I(V) curve shows an exponential increase from 0 V (= the Fermi
energy) to both positive and negative bias voltages. Positive andnegative voltage sides denote sample and tip density of states, respectively, i.e., the tip has no gap around the Fermi energy, ensuring thatthe flame-etched W tip apex has metallic property.
The flames etched W tips were also examined by using the
air STM (JEOL-4200). The tip detected tunneling current without
a crash even in air (see supplementary material, Fig. S4). This provesthat the oxidization on the apex was not significant.
Flame etching mechanism through WO3 generationand vaporization
Although we demonstrated the usefulness and convenience ofthe flame etching to change the W wire to the tip shape, there is stillcuriosity in the mechanism itself. One might think that the etchingoccurs by vaporization of the W atoms from the surface. However,this cannot be the major driving force. This is because W has an

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FIG. 3. (a) SEM images of a W tip
apex fabricated before (upper panel) andafter (lower panel) the swift flame etching (∼0.1 s) at L = 5 mm by shaking one
hand. The tip etched already with LP+O2
gas flame at L ∼ 5 mm for ∼2 s (∼2300 K)
was used. (b) Field emission curves
obtained from the flame-etched tips byshaking one hand. We tested threedifferent tips (red, blue, and black).
The fitted Fowler-Nordheim plots (solidlines) show the tip radii of 20, 45, and52 nm, respectively. The inset shows
the W tip and the counter filament electrode inside UHV setup. (c) An STM
topographic image (raw data, no filtering, only local plane background subtraction) obtained on a fcc-Au(110) surface at 300 K in UHV by using theflame-etched W tip (35 × 17 nm2 ,V s = −90 mV, It = 1 nA). The lower panel
denotes the line profile along the whitearrow. (d) The enlarged STM image ofthe boxed area in (c) (3 × 3 nm2 , V s= −9 mV, It = 1 nA; raw data, no filtering, only local plane background subtraction). The lower panel denotes theline profile along the arrow. (e) Tunneling current as a function of the sample
bias voltage [I(V) curve] obtained on theAu surface showing no gap around the
Fermi energy.

extremely high melting temperature (3695 K) [see Fig. 1(d)] and very
low vapor pressure: 10−8 Torr even at ∼2400 K,30 which is comparable to that of the LP gas flame at L ∼ 5 mm [see Fig. 1(b)]. Additionally, in UHV, the 1500–2300 K annealing (30–40 W) removes theoxide films covered on the W tips apex, while the powerful >2300 K
annealing (50–60 W) makes the apex radius larger (see Ref. 19), i.e.,
the annealing in UHV enhances the diffusion of the W atoms at
the apex and makes the tip apex blunter. However, in air, the flame
etching indeed changed the wire edge to the tip shape, indicatingprecursor reactions between W atoms and air could be necessary forthe flame etching.
The mechanism of the flame etching of the W wire was understood while testing the etching process at different flame positions.
The results in Figs. 1–3 were obtained at L = 5 mm, but here weput the wire at L = 35 mm (the oxygen-rich outer flame with lower
temperature: around the center ∼1500 K, while the outside ∼1200 K[see Fig. 1(b) and Ref. 28]). Still we observed strong flashing, and
the W wire apex formed a tip shape (see supplementary material,
Fig. S2a). However, we found visually that the W wire surface wascoated by yellowish powders. Because the powders were easily peeled
off, we collected them and the powder XRD data were obtained, as
shown in Fig. 4(a) (L = 35 mm). There is in agreement between
the yellowish powders and monoclinic WO3 XRD reference data

Rev. Sci. Instrum. 90, 063701 (2019); doi: 10.1063/1.5085251
Published under license by AIP Publishing(No. 80056-ICSD), e.g., the XRD intensity peaks at 2θ = 23.7○ , 34.3○ ,and 50.1○ correspond to (020), (201), and (140) crystalline planes,
respectively.31 Therefore, the obtained yellowish powder on the W
surface can be identified as WO3 . This is supported by the fact that
the WO3 is generated at ∼1200 K in oxygen atmosphere on the W
surface.32 Enlarged SEM images of a W tip etched at L = 35 mm
are shown in Fig. 4(b). For every ∼0.5 mm from the tip apex, we
separated four regions, namely I, II, III, and IV. Regions II and III
have ∼1 µm size domains in agreement with the reported size of∼0.8 µm,32 and domain size becomes large and scratches are also
observed in region IV. These features indicate that the WO3 film isthicker apart from the apex. Even visually, we observed that region
I has metallic color, while regions II, III, and IV have a yellowish
color.
More detailed information on the amount of the WO3 powder was obtained by SEM-EDX measurements. Figure 4(c) showsenergy dispersive X-ray (EDX) spectra obtained at regions I, II, III,and IV (flame etching at L = 35 mm). A clear oxygen peak was
observed with two W peaks. However, the oxygen peak becomes
small at region I. EDX quantitative analysis results are summarized
in the table in Fig. 4(d), where oxygen is one the majority elements(W: 18%–27% and O: 56%–73%) at regions II, III, and IV, while W
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FIG. 4. (a)–(d) Tungsten trioxide (WO3 )generation by lower temperature flame(1200–1500 K) at L = 35 mm. (a) X-ray
diffraction results of the obtained yellowish powders on the W wires (black line)
and monoclinic WO3 as a reference (redline). The inset denotes WO3 structure
model. (b) A SEM image of the flameetched W tip at L = 35 mm. Enlarged
SEM images at regions I, II, III, and IV.
(c) SEM-EDX spectra obtained at theregions I, II, III, and IV. (d) Percentagesof the composite materials at regions I, II,
III, and IV. (e) Upper panel: SEM image
obtained at the apex of the W tip flameetched at L = 5 mm. Lower panel: SEMEDX measurements obtained along theline in the SEM image.

of oxygen drastically decreases in region I (in the EDX spectrum
obtained at region I, one might note that the carbon peak increased,
but this could be due to the background sample plate behind the
W tip). These experimental results strongly suggest that (1) WO3
is indeed generated around 1200 K32 during the flame etching, andhowever, (2) around the center of the flame at L = 35 mm (∼1500 K)
where the tip apex (region I) is located, the WO3 vaporization rate
is higher than the WO3 generation rate, reducing the oxide film
thickness.
The yellowish WO3 generation was also confirmed using further lower temperature flames. When we put the W wire into the
LP+O2 gas at L∼50 mm, no flashing occurred and no tip shape wasobtained (see supplementary material, Fig. S2b). However, the whole
W surface exposed in the flame was coated by the same yellowish
color, i.e., the W surface could be approximately ∼1200 K, at which
temperature the WO3 vaporization rate is smaller than the WO3generation rate.

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Here, we emphasize that such yellowish WO3 films were notobserved for the flame etching at L = 5 mm with the O2 +LP
flame shown in Figs. 1–3 (∼2300 K). The important point is that
the WO3 has extremely high vapor pressures at ∼1900 K: 0.001
Torr at ∼1300 K, 10 Torr at ∼1700 K, 100 Torr at ∼1800 K, and
760 Torr at ∼1900 K,30 i.e., with the LP+O2 flame, the WO3 vaporization rate is enhanced and therefore at L = 5 mm, the generated WO3
is almost completely vaporized, producing a bare W surface inside
the flame (WO3 melting point: 1746 K30 ). The demonstrations of thelower-temperature flame etching at L = 35 mm confirmed that WO3generation and vaporization could be the key to producing the tip
shape and suggested small amounts of WO3 around the apex owing
to the higher vaporization rate by higher temperatures at the center of the flame. Namely, the thickness of the oxide films around theapex could be negligibly small for the flame-etched W tip obtainedat L = 5 mm. Thus, SEM-EDX measurements were performedat the apex region (∼10 µm), fabricated using the highest flame

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FIG. 5. Schematic illustrations, representing W tip formation by flame etching of a W wire with LP+O2 gas
flame. (a) Flame etching setup of a W
wire with LP+O2 gas flame. The color
shows temperature distribution of theflame. (b) Close-up view of the W wireedge in (a), representing temperaturedependent WO3 generation and vaporization. Gray balls represent W, while
yellow balls represent WO3 . [(c) and (d)]Tip formation at the W-wire edge viewed
from (c) y- and (d) z-directions in (a).
Top (bottom) panels in (c) and (d) show
shape of the W wire before (after) the
etching inside the flame.

temperature [L = 5 mm, see Fig. 4(e)]. The lower panel in Fig. 4(e)shows EDX signals along the red line in the SEM image in Fig. 4(e).
The apex of the flame-etched W tips at L = 5 mm consists of 80% W
and 20% O. The O percentage decreased from ∼30% [Fig. 4(d)]to ∼20% [Fig. 4(e)]. Although precise amounts of the remained
WO3 around the apex cannot be determined by the SEM-EDX, theremained WO3 around the apex is not serious for use in STM/STS
probes and FE sources (as demonstrated in Fig. 3).
Based on these results, the mechanism of flame etching wasmodeled as in Fig. 5, which simply sketches how the flame etching produces a tip shape through WO3 generation and the subsequent vaporization. By inserting the W wire into the center of the
LP+O2 gas flame at L = 5 mm, the temperature around the wire edge
increases depending on the position due to the temperature gradientin the flame [Fig. 5(a)]. This leads to different rates of WO3 generation and the following vaporization, as shown in Fig. 5(b), which
we suggest is crucial to produce a tip shape, as follows. Around the
body of the wire [region I in Fig. 5(b)], the W surface oxidizes tobe WO3 , while the vaporization rate is negligibly small due to the
low temperature (approximately 1200 K). Therefore, the dominant
process is WO3 generation, and the wire surface is only coveredby the WO3 powder without being etched (reducing the volume of
the wire), as shown in Figs. 5(c) and 5(d). Conversely, around thewire surface nearer to the flame center [region II in Fig. 5(b)], WO3
is not only generated but also starts to be vaporized with higher
temperature. This means that the surface starts to be etched. By
increasing the temperature of the W surface from region II to region
III in Fig. 5(b), the corresponding vaporization rate also increases.
Because the vaporization rate, which is given by Arrhenius reaction
law, should increase exponentially with temperature, the vaporization at the wire edge (region III) where the temperature is increased
up to 2300 K should be dramatically enhanced, and the rate should
be limited by the WO3 generation process. Therefore, at region III,
the WO3 generated should be immediately vaporized, and thus theRev. Sci. Instrum. 90, 063701 (2019); doi: 10.1063/1.5085251
Published under license by AIP Publishingetching rate is also enhanced to successfully produce the tip shape[Figs. 5(c) and 5(d)]. We also suggest that such simultaneous W oxidation and vaporization at the flame center are essential to eliminate
WO3 from the wire surface, by which the almost bare W tip apex canbe obtained.
One should note that this flame etching method is performed
in air. Therefore, even once the bare W surface is exposed inside
the flame, the surface is slightly oxidized. Because the WO3 is produced around 1200 K, it is better to pull out the tip from the flame
as fast as possible. This is one reason that the flame etching byshaking one hand produced a sharper tip with a thinner oxide film
coating.
CONCLUSIONSA W tip was found to be fabricated in air within a few seconds just by placing the W wire into the O2 +LP gas flame (∼2300 K)
through the microtorch, which is very fast and convenient, and beneficial for mass production (100 W tips within 30 min, and no expensive setup is necessary). First, the W wire was placed inside the flamefor t ∼ 2.5 s (strong flashing occurred). Then, the wire apex diameter was reduced from 0.3 mm to 0.1 mm. Subsequently, shaking
the wire by one hand, we placed the wire apex only ∼0.1 s inside
the flame. We repeated this several times (total flame etching time:t ∼ 2.7 s) and obtained a tip-shape apex. The obtained W tip provided atomically resolved STM images. Also, since field emission
electrons were detected by applying the electric field of ∼106 V/m,the tip can be used as a FE-SEM source. Tunneling spectroscopy
curves showed no gap at the Fermi energy (i.e., metallic property),and X-ray and SEM-EDX measurements revealed that WO3 generation and simultaneous vaporization at the W surface inside the flame
are the driving force to fabricate the apex with a tip shape. Thus, theflame-etched W tips could be very useful for STM or AFM q-plus
sensor probes working in air or liquid conditions.

90, 063701-7

Review of
Scientific Instruments

SUPPLEMENTARY MATERIAL
See supplementary material for Fig. S1—Flame etching processes of nine W wires (No. 1–9) obtained at the total flame etching time of 0.5 s, 1.0 s, 1.5 s, 2.0 s, 2.5 s, 3.0 s, 3.5 s, 4.5 s, 5.5 s,
7.0 s, and 9.0 s. Total 99 SEM images; Fig. S2—Flame etchingfor the W wire at L = 35 mm and 50 mm; Fig. S3—Flame etching for PtIr, Ni, and Ta wires; Fig. S4—STM measurements in
air using the flame-etched W tip; and Supplementary video during the W tip flame etching: http://adv.chiba-u.jp/nano/yamadaupload/20171130_flameetching.mp4.
ACKNOWLEDGMENTS
This work was supported by JSPS KAKENHI (Grant Nos.
25110011, 15K13357, 16K17521, 16K05887, 17H05353, and17K19023). E.I. and T.K.Y. acknowledge financial support from the
Murata Science Foundation and Hitachi Metals—Materials Science
Foundation. We thank Mr. Ryohei Nemoto, Mr. Kenta Yokota,
and Mr. Masataka Yamaguchi for technical support of field emission curves and STM measurements. We acknowledge Mr. Ryohei
Nemoto, Mr. Nana K. M. Nazriq, and Ms. Kumi Kobayashi forcareful reading of our manuscript.
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