Nanoscale Dipole Antennas Based On Long Carbon Nanotubes Maggie Yihong Chen1,*, Dongning Yuan2, Jie Liu2, and Xuliang Han3

1Omega Optics, Inc. USA 2Duke University, USA 3Brewer Science, USA

Abstract — In this paper, we present the grow of mm-long, well-aligned individual single wall carbon nanotubes (SWNTs) on flat substrates suitable for device fabrication. We also demonstrate the feasibility of using such long nanotubes as nanoscale antennas. To fabricate such antenna structure, well separated and well aligned individual nanotubes were grown by adopting a newly developed method to pattern catalyst islands with uniform size catalyst nanoparticles. The length of nanotubes can be precisely controlled by chemical etching using oxygen plasma treatment at desired locations. Dipoles were fabricated on individual long nanotubes using photolithography method.

Keywords — carbon nanotube, nanoscale antenna, semiconducting carbon nanotube, metallic carbon nanotube, dipole

I. INTRODUCTION

Carbon nanotube, a new carbon material, was discovered by Dr. Sumio Iijima inside the carbon electrode after arc discharge using a transmission electron microscope in 1991 [1]. After more than 10 years of extensive research on carbon nanotubes, it is widely recognized that single-walled carbon nanotubes (SWNTs) have considerable potential as building blocks in future nanoscale electronics due to its unique electronic properties including the high mobility of carriers with the nanotube [2]. However, until recently, the domain of nanoelectronic devices, including devices made from nanotubes, nanoparticles and nanowires, belonged to the domain of wired electronics. Wireless devices exploiting the unique electronic properties of these nanomaterials are rare, due to the negligible coupling of nanometer scale devices to microwaves. The reason for such weak coupling is that the microwave wavelength (centimeters) is much larger than the device size. This size mismatch prevented the society of wireless electronics to benefit from the fast development of nanoelectronics. Fortunately, a recent series of breakthrough discoveries in materials synthesis have resulted in the ability to synthesize carbon nanotubes with length similar to the wavelength of microwaves and RF waves, of order several millimeters to several centimeters in length [3, 4, 5, 6]. These systems may now allow, for the first time, the possibility of wirelessly coupling microwaves from free space into nanoscale electronic devices and sensors [7]. This opens the door to many promising applications in Radio Frequency Identification (RFID), wireless biosensors, and microwave materials such as electromagnetic (EM) shielding and stealth materials.

Although nanotubes are commercially available now, most CNTs are in the form of tangled bundles, which makes it hard to fabricate individual devices. One of the key problems for making such antenna is to obtain well separated individual nanotubes with suitable length for device fabrication.

There is no demonstration of nano-scale antennas due to challenges in synthesis, characterization, electronic contact and fabrication. In this paper, we will present the fabrication of nanotube-based dipole antenna.

II. SYNTHESIZE OF ALIGHED LONG NANOTUBES

It is important to control the density of the long nanotubes for the proposed antenna devices. Several types of catalysts were tested, including Fe nanoparticles prepared using the block copolymer method reported by our group and raw HiPCO nanotubes. In our previous studies, we have concluded that the growth of long nanotubes requires the nanotubes to grow above the substrate surface without touching the substrate, this was achieved by us using the fast heating method and by placing catalysts on a high platform above a substrate reported by peter Burke’s group [4] without the need of “fast-heating” step. Both method can be explained using the proposed mechanism we have reported [5]. More recently, we have discovered that using Raw HiPCO nanotubes as shown in Figure 1, long and aligned nanotubes (in Figure 2) can be grown without the need of “fast-heating” step, the reason is that the raw HiPCo nanotube contains a large amount of elevated metal nanoparticles [8] after deposited on the substrate. These nanoparticles can be used to grow long nanotubes since the raw HiPCO nanotube samples provide a 3D structure where new nanotubes can be grown from these elevated nanoparticles above the surface of the substrate. More interestingly, it was found that if repeated growth were performed on the same sample, more aligned nanotube can be grown from the same catalyst. Additionally, it was found that high flow rate may not be needed for the growth of aligned nanotubes. One preliminary data showed that very slow flow rate can also prepare aligned nanotubes.

Figure 1. TEM image of Raw HiPCO nanotubes from reference 8.

1-4244-0608-0/07/$20.00 © 2007 IEEE. 846

Proceedings of the 7th IEEEInternational Conference on Nanotechnology

August 2 - 5, 2007, Hong Kong

Figure 2. Aligned nanotubes using raw HiPCO nanotubes as catalysts.

III. CHARACTERIZATION OF LONG NANOTUBES

Among all the grown nanotubes, there are semiconducting ones and metallic ones. Before we can make any device out of a wafer, we characterize the nanotubes with Raman spectrum. We characterized five wafers with longest nanotubes around 2mm, and one wafer with longest nanotubes around 3mm. On most of the wafers we found semiconducting ones, with the typical Raman spectrum shown in Figure 3. From its radial breathing mode (RBM) location at 300 cm-1, as well as the shape of its G mode, it is a semiconducting single-walled carbon nanotube with diameter around 0.8 nm.

0

1000

2000

3000

4000

5000

6000

100 125 150 175 200 225 250 275 300 325 350

Raman Shift (1/cm)

Inte

nsi

ty

(a)

0

1000

2000

3000

4000

5000

6000

1200 1300 1400 1500 1600 1700 1800

Raman Shift (1/cm)

Inte

nsi

ty

(b)

Figure 3. Raman spectrum of a semiconducting nanotube.

On one of the wafers, we found a metallic nanotube with the RBM peak at 212.888 cm-1 (Figure 4). But the shape of its G mode does not look like a single-walled carbon nanotube. It is possible that this metallic tube is an inner tube of a multi-walled carbon nanotube.

0

500

1000

1500

2000

2500

100 150 200 250 300 350 400

Raman Shift (1/cm)

Inte

nsi

ty

(a)

0

100

200

300

400

500

600

700

800

1200 1300 1400 1500 1600 1700 1800

Raman Shift (1/cm)

Inte

nsi

ty

(b)

Figure 4. Raman spectrum of a metallic nanotube.

The SES picture of nanotubes around 5mm is illustrated in Figure 5(a). However, the Raman spectrum (Figure 5(b)) shows amorphous carbon performance, which may comes form the residue on the wafer surface during growth.

(a)

847

Raman Excitation at 632.8 nm

0

100

200

300

400

500

600

700

1200 1300 1400 1500 1600 1700 1800 1900 2000

Raman Shift (1/cm)

Inte

nsi

ty

(b)

Figure 5. SEM picture and Raman spectrum of the 5mm long nanotubes.

IV. FABRICATION OF DIPOLE ANTENNA

In order to fabricate a nanoscale dipole antenna, we will first pattern catalyst islands using standard lithography, as illustrated in Figure 6. Each island will be 5 micron square and separated by 100 microns. By controlling the growth conditions, we can grow one long nanotube on each island with their orientation controlled by the gas flow. After the Raman Spectrum characterization, the length and location of the metallic long nanotubes can be determined and electrodes can be patterned using photolithography for antenna feeding.

Then nanotubes will be coated with photoresist again and developed so that a small section between the electrodes can be exposed to oxygen plasma treatment. After treating with oxygen plasma for a short time (3-5 minutes), the exposed nanotube section will be burned out and cutting the nanotubes into desired nanoscale antenna geometry which is shown in Figure 7.

5µm 5µm 5µm

100µm 100µmcatalyst islands

electrode

SWNT

50µm

5µm 5µm 5µm

100µm 100µmcatalyst islands

electrode

SWNT

50µm

Figure 6. SWNT growth method and electrode pattern for antenna probing.

electrode

SWNT

200um x200um

100um

Get rid of the part in between

Figure 7. Nanoscale dipole antenna.

The electrode was patterned on a wafer with metallic nanotubes. The nanotubes can be seen clearly between the patterned electrodes as shown in Figure 8. After expose the middle section of the nanotubes between the electrodes, the dipole is ready under test.

Figure 8. Fabricated electrode pattern on carbon nanotubes.

ACKNOWLEDGMENT

This work was supported by the US ARMY grant W911NG-06-C-0116.

REFERENCES

[1] http://www.nec.com/global/features/worddefiner/key.html#cnt[2] P. McEwen, Physics World, vol. 13, pp. 31, 2000. [3] S. Li, Z. Yu and P. J. Burke, “Electrical Properties of 0.4 Cm Long

Single Walled Carbon Nanotubes,” Nano Letters, vol. 4, pp. 2003-2007, 2004.

[4] Z. Yu, S. Li and P. J. Burke, “Synthesis of Aligned Arrays of Millimeter Long, Straight Single Walled Carbon Nanotubes,” Chemistry of Materials, vol. 16, pp. 3414-3416, 2004.

[5] Huang, S. M.; Woodson, M.; Smalley, R.; Liu, J., “Growth mechanism of oriented long single walled carbon nanotubes using "fast-heating" chemical vapor deposition process”, Nano Letters, vol. 4, pp. 1025-1028, 2004.

[6] Zheng, L. X.; O'Connell, M. J.; Doorn, S. K.; Liao, X. Z.; Zhao, Y. H.; Akhadov, E. A.; Hoffbauer, M. A.; Roop, B. J.; Jia, Q. X.; Dye, R. C.;

848

Peterson, D. E.; Huang, S. M.; Liu, J.; Zhu, Y. T., “Ultralong single-wall carbon nanotubes”, Nature Materials, vol. 3, pp. 673-676, 2004.

[7] P. Burke, Z. Yu and S. Li, “Quantitative Theory of Nanowire and Nanotube Antenna Performance”, cond-mat/0408418, 2004.

[8] Chiang, I. W.; Brinson, B. E.; Huang, A. Y.; Willis, P. A.; Bronikowski, M. J.; Margrave, J. L.; Smalley, R. E.; Hauge, R. H., “Purification and

Characterization of Single-Wall Carbon Nanotubes (SWNTs) Obtained from the Gas-Phase Decomposition of CO (HiPco Process)”, Journal of Physical Chemistry B, vol. 105, pp. 8297-8301, 2001.

849