Electron-bombarded 〈110〉-oriented tungsten tips for stable tunneling electron
emission
T. K. Yamada, T. Abe, N. M. K. Nazriq, and T. Irisawa
Citation: Review of Scientific Instruments 87, 033703 (2016); doi: 10.1063/1.4943074
View online: http://dx.doi.org/10.1063/1.4943074
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REVIEW OF SCIENTIFIC INSTRUMENTS 87, 033703 (2016)

Electron-bombarded 〈110〉-oriented tungsten tips for stable tunneling
electron emission
T. K. Yamada, T. Abe, N. M. K. Nazriq, and T. Irisawa
Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho,Inage-ku, Chiba 263-8522, Japan(Received 24 August 2015; accepted 18 February 2016; published online 11 March 2016)
A clean tungsten (W) tip apex with a robust atomic plane is required for producing a stable
tunneling electron emission under strong electric fields. Because a tip apex fabricated from a wireby aqueous chemical etching is covered by impurity layers, heating treatment in ultra-high vacuum
is experimentally known to be necessary. However, strong heating frequently melts the tip apex andcauses unstable electron emissions. We investigated quantitatively the tip apex and found a useful
method to prepare a tip with stable tunneling electron emissions by controlling electron-bombardment
heating power. Careful characterizations of the tip structures were performed with combinationsof using field emission I–V curves, scanning electron microscopy, X-ray diffraction (transmitted
Debye-Scherrer and Laue) with micro-parabola capillary, field ion microscopy, and field emission
microscopy. Tips were chemically etched from (1) polycrystalline W wires (grain size ∼1000 nm)
and (2) long-time heated W wires (grain size larger than 1 mm). Heating by 10-40 W (10 s) was
found to be good enough to remove oxide layers and produced stable electron emission; however,around 60 W (10 s) heating was threshold power to increase the tip radius, typically +10 ± 5 nm
(onset of melting). Further, the grain size of ∼1000 nm was necessary to obtain a conical shape tip
apex. C 2016 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4943074]

INTRODUCTION

Many sharp and robust tips have been fabricated byusing tungsten (W), which is remarkable for its high melting
temperature of 3653 K and extraordinary hardness. Such
W tips were first used as an electron source of fieldemission microscopy (FEM) in ultra-high vacuum (UHV).1
By introducing inert gas, in 1950s, atomic structures of these
W tips were successfully visualized.2 Today, W tips are used
for a variety of purposes: as an electron beam source forfield-emission-type scanning electron microscopy (SEM),3 asa probe for scanning tunneling microscopy (STM) as well as
atomic force microscopy (AFM),4 as an electron beam source
for low-energy electron diffraction (LEED),5 as an ultrafast
electron pulse source,6 and as a substrate tip for metal film
coatings such as gold7 and magnetic films,8 because W does
not mix with the coated metals.
Atomically sharp W tips have been developed intensively
during the last decades, and many techniques have beenreported. Starting from the aqueous etching,9–13 further treatments in UHV have been studied, such as self-sputtering,14,15
field evaporation,16,17 field-assisted etching,18,19 ion milling,20
and controllable crashing.21 STM or SEM require not only
satisfactory spatial resolution owing to sharpening of the tip
apex but also stable tunneling electron emission, which isdetermined by the local density of states (LDOS) near the
Fermi energy at the surface of the tip apex. Because local
atomic symmetry at the tip apex determines the apex LDOS
differently from the bulk bcc tungsten density of states (DOS),a tip with unchanged atomic morphology and electronic
states under the application of strong electric field must benecessary for obtaining a stable electron source. We show

how unstable electron-emission tips cause troubles in STM
and scanning tunneling spectroscopy (STS) measurements.
Figures 1(a)-1(d) show the STM measurements performed ona clean and atomically flat Cu(111) surface with stable andunstable W tips. During the STM, a strong electric field of
106-1010 V/m is typically applied between the tip and sample.
With stable tips, emission of tunneling electrons from the tip
is maintained at constant level during the STM measurements.
Therefore, a clear topographic image as well as a reliable dI/dV
spectroscopic curve is obtained, as shown in Figs. 1(a) and
1(b), in which atomic terraces and Shockley-type surface statearound −400 meV, respectively, can be observed. On the other
hand, an unstable tip yields time-variable tunneling electron
emission. Therefore, the obtained topographic image shownin Fig. 1(c) reveals several tip-changes (marked by the arrows)
and many drops from the tip (marked by the circles). The dI/dV
curve, shown in Fig. 1(d), reveals spiky noise in the curve (asudden movement of the apical atom changes the tip-sample
distance; thus, the dI/dV value jumps, dI/dV ∝ ρ exp(−2κz),where ρ: LDOS and κ: decay coefficient, and z: tip-sample
distance), owing to very strong electric fields. Importantconsiderations for achieving stable tunneling electron emission are (1) removal of impurity films, such as oxide films,
with which the W tip apex is coated after aqueous etching and(2) construction of a robust apex crystalline structure (atomic
planes) yielding stable LDOS at the apex (sufficiently toughto withstand strong electric fields). Atomically sharp tips may
not be good for achieving stable tunneling electron emission
because adatoms or atomic steps on the apex could be movedby the strong electric field due to weaker coordination.
Effective treatments for removal of the oxide films at
the W apex have been reported: hydrofluoric acid etching0034-6748/2016/87(3)/033703/7/$30.00
87, 033703-1© 2016 AIP Publishing LLC
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emission I–V curves, SEM, X-ray diffraction (transmitted
Debye-Scherrer and Laue) with micro-parabola capillary, field
ion microscopy (FIM), and FEM and found that electronbombardment heating by 10-40 W (10 s) was good enough toremove oxide layers and produced stable electron emission;
however, around 60 W (10 s) heating was the threshold power
to increase the tip radius, typically +10 ± 5 nm (onset of
melting). Further, the grain size of ∼1000 nm was necessary
to obtain a conical shape tip apex.

EXPERIMENTAL

FIG. 1. STM topographic images and obtained dI/dV curves on a clean andatomically flat Cu(111). (a) and (b) were obtained with a stable W tip and(c) and (d) were obtained with an unstable W tip. (a) and (c) show STM
topographic images (50 × 50 nm (a) Vs = −1 V, I = 350 pA, (c) Vs = +2 V,
I = 150 pA), and (b) and (d) show the dI/dV curves (Vs = 1 V, I = 1 nA). In(c), the arrows and circles denote tip-change positions and drops from the tip,
respectively.

in air, self-sputtering in UHV, and proper heating in UHV.
Hydrofluoric acid etching is useful for removing very thickoxide films (several tens of nm).22 This treatment is performed
in air, and the etched tip is exposed to air; thus, slightoxidization of the tip surface cannot be avoided. By using
self-sputtering, the apical oxide films can be removed because
ions that are accelerated toward the apex preferentially hit the
tip part located ∼10 nm from the apex, creating a neck andeventually removing the apex (∼10 nm in size).15,23 Because
this process is performed in UHV, the obtained W apex isbare tungsten, while the surface structure is affected by the
sputtering; thus, the tip apex atomic plane and LDOS are
unclear. Another effective treatment for removal of the apicaloxide films is heating. Heating to 1000-2200 K in UHV has
been reported.16,20,24,25 However, measuring the temperature
at the tip apex is very difficult; thus, the reported heating
temperatures vary from reference to reference. According to
the empirically known resistive heating process,16 first a W tip
is contacted or spot-welded with a conductive wire, following
which a current is flown through the wire. By monitoring theglow of the wire, the tip temperature can be controlled (darkred glow corresponding to 900 K, orange glow corresponding
to 1175-1300 K, yellow glow corresponding to 1375 K),16
while the apex temperature remains unknown.
In this study, we investigated the heating treatment ofa W tip apex in UHV, for removal of impurity films fromthe tip apex surface and for generating robust atomic planes
that can endure strong electric fields for obtaining stable
tunneling electron emission. Characterizations of the tip apex
were carefully performed with combinations of using field

The W tip was prepared in a simple and quick manner, as
follows. In air, the tip was etched from a W wire with aqueous
KOH and cleaned by HF acid. Subsequently, the tip was
introduced into an UHV chamber. Tips were chemically etchedfrom (1) polycrystalline W wires and (2) long-time heated
W wires. All X-ray measurements were performed in air.
STM and STS were performed in an analysis chamber of ourhome-build UHV-STM setup (5 × 10−8 Pa), while tip-heating,field emission I-V curves, and FEM/FIM measurements were
performed in a preparation chamber of the same UHV-STM
setup; thus without breaking UHV, we checked the tip apex.
The shape of the tip apex was checked by using a commercial
SEM setup (Tiny SEM, Technex, 10−3 Pa).
In this study, to check the stability of the electron emission
from the tip apex, field emission I-V curves were measured(sharper tips emitted electrons at lower voltages). Figure 2(a)shows our field emission curve measurement setup, locatedinside the preparation chamber of our UHV STM setup. The
filament (W, diameter of 0.15 mm) was degassed well until
the pressure kept the base pressure during the current flew. The
field emission curves are useful for measuring the radii inside
the UHV preparation chamber in the STM setup. In UHV, acounter electrode (W filament) was set 1 mm away from thetip apex. Field-emitted electrons were detected by the counterelectrode (I–V curve), when electric field in the 106–107 V/mrange was applied between the anode filament and the cathodetip. The maximal negative applied bias voltage was 3 kV, i.e.,
tip radii of up to 150 nm could be checked, while FIM/FEM
allows measuring only sharp tips (r < 10 nm). Bias voltages
above 3 kV frequently damaged the apexes by sparking
owing to the tip apex contaminations. Our LabVIEW program
stopped the voltage application when the counter electrodedetected the set current of 5 nA, and automatically saved thedata. The two I-V curves obtained from the same tip apex
are shown as dots in Fig. 2(b), which were compared with
the Fowler–Nordheim equation.15 By following Ref. 15,
field emission current canbe described as I = 1.537
1.5
2 2
ω(y)),
t (y) = 0.9967 + 0.0716y
× 10−14 × 2πr F2 exp( −0.683φ
F
φt(y)
2
+ 0.0444y , ω(y) = 1.0029 − 0.1177y + 1.1396y2 − 0.2561y3,
y = 3.79F1/2/φ, F = V/5r, where d, distance between the apex
and the counter electrode, 1 mm, and r, tip radius. We assumed
the work function of φ ∼5.5 eV for a W(110) plane.26 Our
I-V measurement setup has a current noise of ∼50 pA, whichis shown as error bars in Fig. 2(b). The simulated I-V curves
are shown as solid lines in Fig. 2(b), where the lower panel isshown in a log-scale. Blue and black lines denote simulated

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FIG. 2. (a) A sketch of our field emission I-V measurement setup in UHV chamber. (b) Black and white dots denote field emission I-V curves experimentallyobtained from the same tip. Black, blue, and red solid lines are simulated I-V curves for r = 18 nm (WF: 5.5 eV), 19 nm (WF: 5.5 eV), and 18 nm (WF: 5.7 eV),
respectively. For sharper tips, the emission curves shift toward lower voltages. (c) A sketch of our electron-bombardment measurement setup without changing
the layout from (a). (d) and (e) show SEM images of a sharp W tip and a melted W tip owing to high heating power, respectively.

lines when the tip radius, r, is 18 and 19 nm, respectively,
with a work function of 5.5 eV. The red line was simulatedwith the same tip radius as the blue line, i.e., 18 nm, but thework function is 5.7 eV. Since the black and the red lines
are almost identical, increasing the tip radius of +1 nm iscomparable to increasing the work function of +0.2 eV. In
this study, we assumed the work function of 5.5 eV,26 since
our X-ray studies confirmed a bcc-W(110) plane at the apex
(the atomic plane larger than 5 nm), and it is hard to believethat the increase of the work function much more than 5.5 eV
at the W(110) apex. From the comparisons in Fig. 2(b), we
can determine the tip radius with an accuracy of ±1 nm, i.e.,
the tip in Fig. 2(b) could have a radius of 18.5 ± 0.5 nm. We
use this method to evaluate the tip radius.
The tips after introducing into the UHV preparation chamber were heated. Figure 2(c) shows our electron bombardment
heating setup. Without changing the tip and the counterelectrode (W filament) positions (the same as Fig. 2(a)),
we passed a current through the filament and applied apositive bias (+1 kV) to the tip, where the filament’s hot
electrons predominantly heat the apex because the electricalflux lines concentrate toward the apex. Heating was controlledwith three parameters: (1) thermal power (W) = hot electrons(mA) × applied positive voltage (kV), (2) time (s), and (3)number of repetitions. During the heating process, we set theconstant voltage, e.g., +1 kV. We changed the filament current
and controlled the emission current from the filament; thus wecontrolled the heating power. For example, at +1 kV, emission
current of 40 mA gives 40 W. We lasted the heating only for
10 s to avoid extra impurities desorbing from the surroundingsof the tip apex. By using the same setup as in Fig. 2(a), we cansmoothly repeat the I–V measurements and heating treatment.
Heating by electron bombardment has been frequently used toincrease the tip temperature up to 2200 K, while a relatively

large heating power was required. Because the filament is
set in front of the tip apex, an infrared pyrometer cannot be
used for precisely measuring the tip apex temperature. The
W tip apex can melt if the heating is too strong. Figures 2(d)and 2(e) demonstrate the results. First, we prepared a sharp
etched W tip, shown in Fig. 2(d), and repeatedly heated it in
UHV, by using a power of 50-100 W for more than 10 s. The
tip was transferred into the STM without halting the UHV
and was approached to the sample. However, no clear STM
topographic images were obtained. Thus, the tip was removed
from the UHV STM and the apex was checked by using SEM.
Figure 2(e) shows the result of this test. The tip apex melted.
With this extra-blunt tip, tunneling current was emitted frommultiple points on the tip apex surface (unstable), and no clear
STM image could be obtained. Therefore, in this study, weinvestigated the proper heating power for removal of impurity
films without making the tip blunter and unstable emission.
To avoid complete melt, in this study, we used a heating timeof 10 s.

RESULTS AND DISCUSSIONFirst, we characterized the tips etched from a commercial
W polycrystalline wire (purity 99.9%). It is known by
FEM/FIM that even polycrystalline wire, the etched tip hasa (110) plane. We confirmed the origin of this (110) plane
appearance by using X-ray measurements. Figure 3(a) showsan X-ray image (transmitted Debye-Scherrer) obtained fromthe W polycrystalline wire. In spite of the polycrystalline, Xray image shows not only Debye-rings but also several spots,
which shows that the W wire had a ⟨110⟩ orientation along thewire axis. The reason could be the wire drawing process. We
chemically etched the wire and obtained the tip (SEM image

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FIG. 3. (a) Transmission type X-ray Debye-Scherrer image obtained from a commercial polycrystalline W wire. (b) X-ray Laue image obtained from the tip
apex by illuminating the 5000 nm region with micro-parabola capillary. The inset shows the tip’s SEM image. All data were obtained by using an X-ray W tube,
40 kV, 20 mA, exposure time of 120 min. During the measurements, the W wire rotated around the fixed z-axis.

in inset to Fig. 3(b)). By using the micro-parabola capillary,
the X-ray was projected only onto the tip apex (∼5000 nm).
The obtained X-ray Laue image (Fig. 3(b)) now shows no
Debye-rings, but sharp spots, suggesting that the tip apex issingle-crystallized, even without any heating, and the ⟨110⟩
direction is parallel to the wire axis, with possible deviation
contained within 5◦. In Fig. 3(b), four spots can be observed(contained within the white circles), indicating four 1000-nmsize grains at the apex. This can explain why the FIM/FEM
images obtained from the tip etched from the polycrystalline
wire exhibit a (110) plane at the apex.8
Next, the tip etched from the polycrystalline wire was set
into the UHV chamber and the field emission I-V curves were
measured. We measured three times as shown in Fig. 4(a);however, the obtained I–V curves varied. This variability canbe attributed to adsorbed molecules or impurity layers on the
tip apex surface. After heating the unstable tip in Fig. 4(a) by
10 W (10 s × 1), the obtained I–V curves in Fig. 4(b) suggested
good reproducibility, indicating that unstable surface layers
were removed and a stable W metal surface was exposed. The
three I–V curves suggested the tip radius of 18 nm. During theheating between Figs. 4(a) and 4(b), the pressure increased
from 10−8 Pa to 10−7-10−6 Pa. If the residual gases were
ionized, the residual gas ions might also sputter the tip apex andtherefore the tip radius could become sharper. We increased
the heating power up to 36 W (10 s × 1) and obtained the I–V
curves identical to those in Fig. 4(b), indicating no change at
the apex. We also tested these heating treatments in several
different setups, but in keeping the tip-sample separation of∼1 mm with W filaments with a diameter of 0.15 mm, wealways get the emission current of 10-40 mA by applying thebias of +1 kV and the filament current of typically 2.0-2.3 A.
These parameters will be useful for many researchers whoneed stable W tips.
Now we are curious to check the threshold power, whichstart to change the apex and increase radius (make blunter)and finally complete melt (like Fig. 2(e)). In Figs. 4(c) and
4(d), we investigated the threshold heating power. Lower
panels show in a log-scale. Solid lines are experimentallyobtained I-V curves, and the dashed lines are fitted curves.
Figure 4(c) shows the I–V curves obtained for another W

tip with the radius of 17 nm after annealing it up to 36 W
(10 s × 1). Again, the I–V curve measurements were repeatedthree times, suggesting good reproducibility and thus stable W
apex fabrication. To determine the threshold power, we further
increased the power. After heating by 40 W, 50 W, and 55 W,
there were no changes in the I–V curves. Each heating cycle
lasted 10 s. We repeated the heating three times (10 s × 3);
however, the I–V curves remained the same. Finally, by using
a 58 W (10 s × 1) annealing power, we managed to increase thetip radius to 26 nm, as shown in Fig. 4(d). We tested several
W tips made from polycrystalline W wires and found thethreshold heating power of ∼60 W for obtaining the tip blunter(typically, by 60 W, the tip radius increases by +10 ± 5 nm).
Our study confirmed that only one heating with a powerof 10–40 W lasting 10 s is sufficient for obtaining a stable W
tip apex, producing a stable electron emission. An additional
finding from this study is that gentle heating at about 60 W
(10 s × 1) can change the apex blunter. Typically, the tip
radius increased with +10 ± 5 nm, which could be importantwhen using the W tips as a substrate for fabricating metalfilm-coated tips because the electronic or magnetic propertiesof the covered films are determined by the film crystalline
structure.27–29 A flat and clean W tip apex is necessary forcontrolling the film properties, which is in contrast to themotivation for producing an atomically sharp tip. An apex
with many atomic defects, such as adatoms, steps, and kinks,
destroys the crystalline structures of the deposited film, which
was experimentally confirmed by FIM, in which the deposited
Fe film on a sharp W tip exhibited amorphous-like structure
instead of a bcc symmetry.8 Advantages of changing tip apex
blunter in a controllable way could be understood by atomic
layer stacking models of the apexes with different radii (bccW(110), atomic layer distance: 0.223 nm), in Fig. 4(e). These
simple models suggest that the apex shape and radius areimportant for controlling the flat and facet areas at the apex.
Here, the “flat” areas are defined as the areas with terraces
exceeding 5 nm. For example, a tip with a 5-nm-long terrace
has a flat area only within a diameter of 4 nm; however, the flatarea diameter increases to more than 30 nm when the radiusincreases to 40 nm, i.e., to obtain stable electron emission, a tip
with r >> 5 nm could be better than an atomically sharp tip.

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FIG. 4. Experimental field emission I-V curves. The tips were etched from a commercial polycrystalline W wire, and subsequently introduced into the UHV
setup. (a) The etched tips exhibited three different curves. (b) After heating by 10 W (10 s × 1), reproducible curves were obtained, yielding the tip radius of
18 nm. (c) Field emission curves obtained from another W tip, yielding the tip radius of 17 nm. The lower panel shows in a log-scale. Solid lines denoteexperimentally obtained I-V curves and the dashed lines denote fitted curves. (d) After heating by 58 W (10 s × 1), the tip radius increased to 26 nm. The lower
panel shows in a log-scale. Solid lines denote experimentally obtained I-V curves and the dashed lines denote fitted curves. (e) Atomic plane models of tip
apexes for different tip radii. Top atomic layer diameters are shown.

We also tested the tip etched from the long-time heated
W wire since the heated wire has larger grains.30 We set thepolycrystalline W wire (diameter of 0.3 mm) of Fig. 3 into the
UHV chamber and electrically heated the wire (76.7 W for45 h). Then, the wire was extracted from the UHV chamber,and the wire cross section was polished in air. The crosssection’s SEM image (Fig. 5(a)) reveals two domains (A andB) with sizes above 100 µm, i.e., the grain size increased as we
expected. We also characterized the crystalline structures bytaking X-ray diffraction image (transmitted Debye-Scherrer)
obtained by rotating the W wire along the wire axis (Fig. 5(b)).
Different from the data in Fig. 3(a), only sharp spots areobserved in Fig. 5(b) (no Debye rings), confirming single
crystallization. We repeated to take several Laue images byilluminating X-rays at different positions (z = 0.0, 0.5, 1.0,
1.5, 2.0, 2.5, 3.0, 3.5 mm) along the z-axis. As shown in
Fig. 5(c), the spot patterns in the Laue images are the samefor z = 0.0 to z = 0.5 mm, but these change from 1.0 mm.
Typically, every 1 mm, spot patterns change, indicating thatthe domain size along the z-axis could be 0.5-1.0 mm.
We etched this wire to obtain a tip. Figure 5(d) shows
the tip’s SEM image. The surface is not smooth and manyedges can be observed. Figure 5(e) shows the top view of the
tip’s SEM image. The tip had a quadrangular pyramid shape.

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FIG. 5. X-ray, SEM, and FIM/FEM results obtained from the polycrystalline W wire (diameter 0.3 mm) heated by 76.7 W for 45 h and its etched tip. (a) SEM
image of a cross section view of the heated wire. Domains A and B are observed. (b) Transmission type X-ray Debye-Scherrer image. (c) X-ray Laue images
obtained at different positions along the z-axis. (d) A W tip, chemically etched from the annealed W wire. (e) SEM image observed from the z-axis. (f) X-ray
Laue image obtained from the tip apex by illuminating the 5.0 µm region with micro-parabola capillary. ((g) and (h)) FIM (He, 3 × 10−3 Pa) and FEM images(15 × 15 nm2), obtained for the same tip at 300 K. All data were obtained by using an X-ray W tube, 40 kV, 20 mA, exposure time of 120 min.

We illuminated the X-ray only at the tip apex (5 µm) with amicro-parabola capillary. The obtained Laue image in Fig. 5(f)shows the spots, revealing that the ⟨110⟩ direction is parallelto the z-axis, with deviation contained within one degree.
These results indicate that grain boundaries are necessary for
fabricating conically shaped tip apexes.
This unique shape tip was set into the UHV preparationchamber and I–V measurements performed. Figure 6(a) shows
three I–V curves obtained after introducing the tip from air
into UHV; all curves are different, similar to the case for the
W tip fabricated from the polycrystalline wire (Fig. 4(a)). As
shown in Fig. 6(b), after annealing by 10–20 W (10 s × 1), therepeatedly obtained I–V curves are identical, suggesting the
tip radius of 31 nm. A tip with rather large tip radius could
be fabricated by using the electrically heated wire. Same as
in Fig. 4, here we tried to determine the threshold heating
power for increasing the tip radius. We found that the tip radius
started to increase when the power exceeded 40 W. In Fig. 6(c),after annealing by 40 W (10 s × 1), the resulting I–V curve
suggested the tip radius of 37 nm (black). We repeated the I–V
measurements. The second curve (red) suggested the tip radiusof 36 nm while the third one suggested the tip radius of 40 nm.
We re-fabricated a tip from the electrically heated wire
and annealed the tip by 20 W in UHV, following which

FIG. 6. ((a) and (b)) Field emission I-V curves obtained from the W tip in Fig. 5, before and after heating by 20 W (10 s × 1). The emission becomes stable after
the heating, yielding the tip radius of 31 nm. (c) Further heating, up to 40 W (10 s × 1), increased the tip radius, while the three curves show 36, 37, and 40 nm.
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we performed FIM and FEM measurements (Figs. 5(g) and5(h)). Although the crystalline orientation was parallel to thebcc-⟨110⟩ direction, the FIM image reveals many 1–3-nmlong atomic steps at the apex, confirming that the apex has
many atomic defects. These tips are not as good as a tip
with a stable tunneling electron emission. Strong electricfields likely modify the apical atomic structure; therefore, theobtained I–V curves in Fig. 6(c) could be changed in every I–V
measurement. Further, if films are deposited on such a tip, thefilm crystalline structure could become amorphous because
no flat atomic planes dominate at the apex.
These results in Fig. 5 suggest that W wires with grain
sizes of ∼1000 nm should be used for obtaining conicallyshaped apexes with atomic (110) plane, instead of using wires
with 0.1–1-mm-size grains, in spite of the fact that the qualityof the ⟨110⟩-orientation increases. Comparing the FIM andFEM images, electrons are found to be emitted from one step
(marked by the dotted circle in Fig. 5(g)).

CONCLUSIONS

Stable W tips, producing constant tunneling electron
emission in the presence of strong electric fields, can be
fabricated by excluding impurity layers at the tip apex andproducing stable LDOS near the Fermi energy owing to therigid bcc-(110) atomic plane. In this study, we investigated
tip preparation methods to obtain the stable W tips. Stabilityof the electron emission from the tip apex was checked bymeasuring field emission I-V curves. W tips etched frompolycrystalline wires have single crystal grains with a sizeof ∼1000 nm. Electron bombardment heating was used toheat intensively at the apex. 10-40 W heating (lasted 10 s)
was required to get stable electron emission; however, heating
more than the threshold power (∼60 W) started to increase
the tip radius (typically +10 nm), which might be useful togrow an atomically flat film with a bcc symmetry (a deposited
film on an atomically sharp tip become amorphous). Also,
grain size of the wire was found to be an important parameterfor determining the shape, i.e., grain size of ∼1000 nm is
necessary to obtain a conical shape tip apex.
ACKNOWLEDGMENTS

Kind support and discussions with Professor Dr. T.
Mizoguchi are acknowledged. This work was supported by

Rev. Sci. Instrum. 87, 033703 (2016)

JSPS KAKENHI Grant Nos. 23681018 and 25110011, Asahi
Glass Foundation.
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