Application of Single-Walled Carbon Nanotube to the Probe of Scanning Tunneling Microscopy

A single-walled carbon nanotube (SWNT) with well-defined structure has a potential as a probe of scanning microscopy. However, SWNT tip has not been applied to practical use yet because of the difficulty in the tip preparation. We have developed a technology for fabrication of SWNT tips with the yield rate of 25%. Various lengths and shapes of SWNT tips were examined as the probe of ultrahigh vacuum scanning tunneling microscopy (STM). We found that the length of SWNT was a crucial factor for the application to STM. Atomic-scale resolution could be obtained on the surface of highly oriented pyrolytic graphite with an SWNT tip shorter than 300 nm. In the case of ring type SWNT tip, which used the side wall of SWNT, the STM images depended on the scan direction due to the half-ring shape, and atomic-scale resolution could be obtained when scanned along the circumference direction of the ring. Although the stability of the SWNT tip during scanning needs to be improved, present results prove the potential of SWNT for STM probes. [DOI: 10.1380/ejssnt.2013.105]


I. INTRODUCTION
A single-walled carbon nanotube (SWNT) is a onedimensional nano-material that consists of rolled-up graphene and has excellent properties usable to numerous applications.One of such applications is the use of an individual SWNT as the probe of scanning probe microscopy.In this case, the structure of probe apex is critical to obtain high resolution.As-grown SWNT by chemical vapor deposition (CVD) has a cap at one end and a catalyst particle at the other end.The SWNT cap has a half-fullerene-like structure that consists of six pentagons.Regions other than the pentagons are flat hexagons, so the portion with the pentagon extrudes.Thus, SWNT is a potentially ideal probe because of the well-defined geometry and the diameters approaching to the size of small organic molecules.Cheung et al. reported fabrication of an isolated SWNT on a probe tip by CVD.The fabrication rate was as low as 10% [1].Chen et al. reported the application of SWNT to a probe of atomic force microscopy, and isolated DNA molecules and gold nanoclusters were imaged by the SWNT tip [2].However, practical applications of an isolated SWNT to a probe tip has not been established because of difficulty in the growth control.In particular, there are few reports on the use of SWNTs to a scanning tunneling microscopy (STM).Inoue et al. used ensemble SWNTs grown on a PtIr tip apex as a sample and measured the electronic density of states (DOS) of individual SWNTs by scanning tunneling spectroscopy (STS) [3].However, an isolated SWNT has not been used as a probe of STM, because it is believed that an SWNT is not stable for vibration and is not useful.In the present study, we examined the SWNT properties as the probe of STM.
We have developed a technology for fabrication of SWNT tips.In our experimental process, because many * Electronic address: j1213701@ed.tus.ac.jp tungsten (W) tips were produced with an automated electropolishing device, we could efficiently investigate the conditions for fabricating an individually standing SWNT on the W tip. We could prepare many SWNT tips stably.Furthermore, the fabrication rate was 25%, twice as high as the preceding study [1].In this paper, we will focus on the application of fabricated SWNT tips to STM.

II. EXPERIMENTAL
An individually standing SWNT was grown on a W tip (average curvature radius of 100 nm).The W tips were mass-produced by electropolishing a W wires (0.3 mm in diameter) with the automated electropolishing device that had paralleled circuits of nanosecond-order switching.To prevent alloying of catalysts with W, the W tip was deposited with an Al film (40 nm thick) by vacuum deposition [4].The Al film was oxidized in air forming Al-oxide, which acted as a barrier for alloying of the catalyst.A Co catalyst film (1 nm thick) was then deposited on the Al layer by vacuum deposition.The W tip was introduced into the CVD furnace at room temperature.Then, the air was evacuated with a rotary pump down to 1.3 × 10 2 Pa and replaced by Ar/H 2 (3% H 2 by volume) with the pressure of 9.3 × 10 4 Pa.The flow rate of the Ar/H 2 gas was 140 sccm (standard cubic centimeter per minute; calibrated using the conversion factor for Ar).The W tip was heated from room temperature to 200 • C in 5 min.Then the temperature was raised to 860 • C in 20 min, and the CVD growth was performed by bubbling liquid ethanol using Ar/H 2 gas at 9.3 × 10 4 Pa.The flow rate of the bubbling gas was about 84 sccm.We observed each W tip by scanning electron microscopy (SEM) and selected ones suitable for the STM tip.The length and shape of SWNT could be arranged by growth time to some extent.Examples of a straight type SWNT tip and a ring type SWNT tip can be found in Fig. 2(a) and Fig. 3(a), respectively.The length of SWNT basically increases with the growth time.A long straight SWNT is not stable and the free end of the SWNT tends to adhere to the W surface [4].Thus, a ring type SWNT can be obtained by extending the growth time.We also observed some of the CVD tips by transmission electron microscopy and confirmed that the grown nanotubes were single-walled with diameters 2-3 nm.For desorption of adsorbates on the tip surface, the SWNT tip was cleaned by annealing in a pressure less than 1 × 10 −4 Pa over night.Then, the SWNT tip was used as the probe of STM and highly oriented pyrolytic graphite (HOPG) was used as a sample.The pressure during STM observation was less than 1 × 10 −4 Pa.In STM observation, the tip was set at 0 V, and the sample was biased.STM images were obtained in the constant current mode.

A. Straight type SWNT tip
Various lengths and shapes of SWNT tips were used as the probe of STM.In the case of a straight type SWNT tip with length of ∼ 1000 nm, stable STM images could not be obtained.We analyzed the line profile of an STM topographic image of a gold film on mica obtained with the long SWNT tip. Figure 1 shows the line profiles of the STM image with different scan rates.At the scan rate of 1 s/1.0 µm, the line profile has a mound shape declining at both ends as shown in Fig. 1(a).It seemed as if the SWNT tip was trapped at the center of scan, and the SWNT axis was bent as the root of SWNT approached both scan ends.At the slower scan rate of 3 s/1.0µm, the line profile exhibits sawtooth shapes as shown in Fig. 1(b).With a surge of the tunneling current, the SWNT tip was suddenly retracted and retuned back to the set point again.The exact reason for the sawtooth behavior is not known, but it reflects some instability of the tunneling current.We thought the origin of instability related to the thermal vibration of a long SWNT.The thermal vibration amplitude increases with the tube length l as l 1.5 .For a 1000 nm SWNT, the thermal vibration amplitude is on the order of 10 nm with frequencies more than 1 MHz [5].
Hence, it is crucial to eliminate or reduce vibration for the application of SWNT to the probe of STM.We fabricated short SWNT tips to reduce the vibration.With an SWNT tip of ∼ 300 nm long, atomic-scale resolution of an STM image could be obtained.However, the tip height was not stable.Tunneling current was easily shot up by bumps and valleys on the sample surface.This result indicates that the length of SWNT should be shorter than 300 nm for the probe of STM.Moreover, when a SWNT tip of ∼ 227 nm long and inclined by 45 • to the W tip axis was used, atomic-scale resolution could not be obtained.Because of the thermal vibration of the SWNT tip, the feedback of the tunneling current was unstable.Thus, it is also important that the SWNT tip is standing up straight on the W tip to minimize the vibration effect.
We fabricated the SWNT tip of ∼ 180 nm long with a bundled root as shown in Fig. 2(a).An STM image of HOPG obtained with the SWNT tip is shown in Fig. 2(b).At the beginning of imaging, the tunneling current was not stable, but it became stable with repeating scans, probably because adsorbed molecules on the tip apex were swept away.Atomic-scale resolution is clearly confirmed in the HOPG image shown in Fig. 2(b).We obtained an STS spectrum from the SWNT tip and derived dI/dV ∝ DOS as shown in Fig. 2(c).The large peaks in the spectrum reflect the Van Hove singularity of the SWNT DOS, although the spectrum is noisy.The dI/dV spectrum showed a voltage gap of ∼ 0.35 V, which is consistent with a feature of a semiconducting SWNT.The diameter of SWNT was estimated ∼ 2.3 nm from the voltage gap [6].However, the measured spectrum should reflect both the electronic states of the SWNT tip and HOPG sample [7].For precise determination of the SWNT DOS, a sample with a constant DOS near the Fermi level is preferable [3].
Here, we should comment on the effect of an Al-oxide between SWNT and W. The STM image and dI/dV spectrum shown in Fig. 2 were not affected by an insulator layer.A stable electrical contact was obtained between the SWNT and W tip. Actually, many SWNTs grew on the Al-oxide surface and they extended to the bare W surface. Furthermore, the root part of the standing SWNT tip could lie on the Al-oxide.Thus, the SWNT tip might electrically contact with W through other SWNTs.

B. Ring type SWNT tip
A ring type SWNT tip can be characterized by the curvature radius.With an SWNT tip with the curvature radius of ∼ 250 nm, the STM images depended on the scan direction.Atomic periodicity was obtained in a scan direction (referred to x-direction), but it was unstable in the perpendicular scan direction (referred to y-direction).These phenomena can be attributed to the shape of a ring type SWNT tip whose both ends are fixed to the W tip apex: the SWNT tip is stiffer in the direction parallel to the tube axis.We assumed that the x-direction scan corresponded to the circumferential direction of the ring type SWNT and the y-direction scan was perpendicular to the circumferential direction.during scanning.Atomic-scale resolution for HOPG was obtained as shown in Figs.3(b) and (c).The difference in resolution due to the scan direction was not significant because of the small curvature radius, although the x-direction scan image [Fig.3(b)] is more stable than the y-direction scan image [Fig.3(c)].Thus, the x-direction scan corresponds to along the circumferential direction of the ring.We obtained STS spectrum from the SWNT tip and derived dI/dV spectrum as shown in Fig. 3(c).The dI/dV spectrum had no gap, indicating it was a metallic SWNT.We observed STM images with various ring type SWNT tips.Regardless of the ring size, resolution was dependent on the scan direction.However, it should be noted that the scan direction-dependent resolution for the ring type SWNT tip is due to the stability to vibration.Though the side wall of SWNT was used for electron tunneling, atomic resolution was obtained with the ring type tip.

C. Comparison of resolution
The above results indicate that STM images obtained by an SWNT tip is dependent on the tip shape.The resolution of STM images was compared between the two types of SWNT tips.The obtained protrusion in the line profile is more than three times larger for the straight SWNT tip than for the ring-SWNT tip.This might reflect the size difference between the tip apex and the side wall of the SWNTs.However, the STM image obtained by the straight SWNT is distorted, not well reflecting the symmetry of HOPG.In spite of the highly sensitive line profile, the scanning stability of the SWNT tip is not enough.The STM image tends to be anisotropic and is extended along the scan direction.The stability of SWNT tip should further be improved by clarifying the origin of instability.For an SWNT tip as the probe of STM, a slight crash of the tip to the sample surface does not damage the tip and sample.An SWNT is mechanically strong and can be bent.The flexibility of SWNT can be utilized mostly.This feature is an advantage, and the SWNT tip has a potential as a probe.

IV. CONCLUSION
We have developed SWNT tips directly synthesized on a W tip and applied them to the probe of STM.The length of SWNT is important for the application to STM.Because a long SWNT tends to vibrate, a shorter SWNT is better for the probe of STM.In the case of ring type SWNT tip, the STM images depended on the scan direction due to the half-ring shape.In the case of straight type SWNT tip with a length shorter than 300 nm atomicscale resolution could be obtained in the STM image of HOPG.Although the line profile of SWNT tip was as good as that obtained by a conventional metallic tip, the stability of scanning was not enough.We could demonstrate the possibility of SWNT tip for the probe of STM and the results of this study would be beneficial for the application of SWNT probes.

FIG. 1 :
FIG. 1: Line profiles of an STM image obtained with the SWNT tip of ∼ 1000 nm long at (a) the scan rate of 1 s/1.0 µm and (b) the scan rate of 3 s/1.0µm.The bias voltage was 2.3 V, and the tunneling current was 30 pA.The black arrows show the fast scan direction.

Figure 3 (
FIG. 2: STM observation results with a straight type SWNT tip.(a) SEM image of an SWNT tip.The primary electron energy was 0.5 keV.(b) STM image of HOPG with the SWNT tip.The scan area was 5 nm × 5 nm, the bias voltage was −0.4 V, the tunneling current was 0.5 nA and the scan rate was 260 ms/5 nm.The white arrow shows the fast scan direction.(c) dI/dV spectrum obtained from the SWNT tip.
Figure 4(a) shows the line profiles of STM topographic images [Figs.4(b) and (c)], which are a part of each image shown in Fig. 2(b) and Fig. 3(b).

FIG. 3 :
FIG. 3: STM observation results with a ring type SWNT tip.(a) SEM image of an SWNT tip.The primary electron energy was 0.5 keV.STM images of HOPG with the SWNT tip in (b) the x-direction scan and (c) the y-direction scan.The scan area was 5 nm × 5 nm, the bias voltage was −0.25 V, the tunneling current was 0.5 nA and the scan rate was 260 ms/5 nm.The white arrow shows the fast scan direction.(d) dI/dV spectrum obtained from the SWNT tip.