En Route to Carbon Nanotube Electronics

By: Professor Moonsub Shim, Material Matters Volume 2 Issue 1

Professor Moonsub Shim Department of Materials Science and Engineering and Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign

Ballistic electron transport, current carrying capacities on the order of 109 A/cm2, and one of the largest known specific stiffness are only a handful of the many exciting and potentially useful properties of single-walled carbon nanotubes (SWNTs).1 Especially in developing areas of electronics such as wearable/flexible electronics and nanoelectromechanical systems, SWNTs may find use as high performance semiconducting as well as conducting elements. However, as with any new material, there are many obstacles to overcome before most envisioned benefits can be realized. Three of the biggest challenges to be addressed in order to integrate SWNTs into high performance electronic devices are; 1) electronic inhomogeneity where a random mixture of metallic and semiconducting SWNTs can degrade device performance, 2) extreme sensitivity to minute changes in the local chemical environment, and, 3) difficulties in aligning and patterning. While the first challenge is specific to SWNTs, the second and the third issues are general to one-dimensional materials. Here, we briefly discuss some of our current efforts in addressing all three challenges.

Electronic Inhomogeneity

The distribution of diameters and chirality in SWNTs produced by current synthetic methods leads to a mixture of metallic and semiconducting characteristics. This electronic inhomogeneity imposes one of the biggest challenges in all prospects for electronic implementations. One obvious but devastating consequence of the electronic inhomogeneity in developing high performance transistors out of SWNTs is that metallic SWNTs will electrically short the semiconducting channels. This appears as a large off-current in the transport characteristics of transistors composed of multiple SWNTs as shown in the uppermost curve in Figure 1A. Recently demonstrated selective chemical functionalization methods may provide a simple scalable means of eliminating metallic tubes from SWNT transistors and electronic devices. We have carried out a combination of electron transport and Raman studies on the reaction of 4-bromobenzene diazonium tetrafluoroborate (4-BBDT, Aldrich 673405) directly with single and networks of SWNT transistors to examine the potential benefits and limitations of this approach.2

Figure 1. A. The electrical response of SWNTs which are the active elements within a back-gated transistor (schematic) upon reaction with 4- bromobenzene diazonium tetrafluoroborate (4-BBDT) of indicated concentrations. B. Reactivity distribution of metallic and semiconducting SWNTs to 4-BBDT as measured by the increasing disorder (D) to tangential (G) band intensity ratio in the Raman spectra. Filled symbols – metallic; Open symbols – semiconducting.

Figure 1A shows the electrical response of a transistor consisting of multiple SWNTs (both metallic and semiconducting) upon reaction with increasing concentration of 4-BBDT. The best case scenario where only metallic SWNTs react and become insulating is achieved at 5 μM. At this concentration, the reduction in the off-current is essentially identical to the decrease in the on-current indicating that only the metallic SWNTs which do not have gate voltage dependent drain-source current have reacted. The key result is that the performance in terms of carrier mobility remains the same when only the metallic SWNTs are chemically turned off.

Figure 1B shows the ratio of intensities of disorder (D) and tangential G (in-plane C-C stretch) Raman modes of several SWNTs upon reaction with 4-BBDT of increasing concentration. The D mode arises from a double resonance process where momentum conservation is achieved by defect (or disorder) scattering. Since 4-BBDT reacts with SWNTs by covalent C-C bond formation, it inevitably breaks pi-bonds of SWNTs. This chemical reaction then induces additional disorder leading to the enhancement of D mode in the Raman spectra of SWNTs. Hence the ratio of intensities of D and G modes provides a simple spectroscopic measure of chemical reactivity. The metallic SWNTs are found to have, on average, higher reactivity towards 4-BBDT. However, there is a considerable overlap between metallic and semiconducting SWNTs. While in a device consisting of a limited number of SWNTs, the ideal situation of selectively turning off metallic SWNTs can be achieved, when there is a large number of SWNTs, a significant degradation of device performance is inevitable and is observed. While reaction with 4-BBDT cannot be applied directly to improve device performance beyond transistors consisting of a limited number of SWNTs, additional factors such as external electrochemical potential to shift the Fermi levels of SWNTs may be exploited to achieve improvements and are being examined.

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Extreme Sensitivity to the Local Chemical Environment

Another key challenge in integrating SWNTs into electronics is the extreme sensitivity to molecular adsorption and to changes in the local chemical environment. Some consequences in terms of electronic applications include difficulties in achieving complementary p- and n-type devices (i.e. doping) due to oxygen adsorption3 and large hysteresis4 associated with interaction with the substrate. Extremely environment sensitive electronic properties of SWNTs can also lead to undesirable and/or unpredictable behavior even with small changes in the surrounding medium. All of these challenges can actually be overcome by exploiting the highly sensitive electronic properties of SWNTs. With simple adsorption of polymers having appropriate chemical groups, electron or hole injection can be achieved.5 Addition of an electrolyte into the host polymer allows electrochemical gating with efficiencies approaching the ideal limit and eliminating hysteresis.6 The short Debye lengths of the electrolyte solution can also screen out many undesirable external effects.

Device characteristics of individual SWNT transistors operating in the back gate configuration are shown in the insets of Figure 2. Notice that all devices show p-channel operation (negative gate voltages) with little or no n-channel conductivity and large hysteresis. The main panels of Figure 2 show the same corresponding SWNTs operating with polymer electrolyte gate. Within the gate voltage range examined, very low leakage current of < 500 pA at relatively large gate voltages and < 100 pA near depletion region (mainly due to non-Faradaic charging currents in electric double layer formation) preserves the high on-off ratio of ~105. More importantly, varying the chemical groups of the host polymer from electron donating to electron withdrawing can tune the doping levels in semiconducting SWNTs while maintaining high carrier mobilities.

Figure 2. Carbon nanotubes gated with polymer electrolytes. Chemical groups of the adsorbed polymers can change initially p-type only devices (insets) into p-, n- and ambipolar devices. Polymers used are poly(acrylic acid) (Mn ~1240) for p-type, poly(ethylene oxide) (Mn = 1000) for ambipolar, and polyethylenimine (Mn ~ 800) for n-type devices .

Polymer electrolyte adsorption provides a simple air-stable means of doping SWNTs in addition to blocking out other external chemical sensitivity via short Debye lengths. However, electrochemical gating is limited in terms of device switching speed since it relies on ionic mobility. Nevertheless, operation speed of several hundreds of Hertz observed in these devices is already useful for simple display applications. Further studies are on-going to develop strategies for enhancing device switching speeds and therefore fully exploiting high carrier mobilities of SWNTs.

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Alignment and Patterning

The final challenge to be discussed here is the difficulties in aligning and patterning SWNTs. Vertical alignment in both multi-walled and single-walled carbon nanotubes have been achieved directly from chemical vapor deposition where the nanotubes grow out perpendicular to the substrate.7 This geometry may be particularly useful for field emission applications. However, for many electronics applications, horizontal alignment along the substrate is desired. One solution involves exploiting interaction of SWNTs with the substrate. On mis-cut single crystal quartz, we have shown that SWNTs grow horizontally aligned along the quartz step edges.8 Figure 3A shows a nearly perfect alignment of SWNTs grown on mis-cut quartz. By patterning catalyst particles (evaporated Fe or solution deposited Fe salts), interesting structures can be obtained as shown in Figure 3B. Where the catalysts are patterned, there is a very high density of SWNTs leading to random orientations. Away from the catalyst areas, SWNTs start to align along substrate step edges. The randomly oriented high density areas are highly conducting and can be used as the “electrodes” to contact the aligned SWNTs acting as the active elements in an all SWNT transistor.

Figure 3. A. Nearly perfect horizontally aligned SWNTs directly grown on miscut single crystal quartz. B. An “all-SWNT transistor” where large density area on top of the patterned catalysts serves as conductors and the aligned SWNTs act as active elements.

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Advances in nanoelectronics continue to be anticipated from the exceptional properties of SWNTs. Most benefits may be gained in developing areas such as flexible electronics and nanoelectromechanical systems where an unprecedented combination of electrical and mechanical properties may be exploited together. We have addressed some of the major challenges to be overcome en route to integrating SWNTs into such devices and systems here.

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Contributions by C. Wang, G. Siddons, and C. Kocabas to this work and collaboration with Prof. J. Rogers are gratefully acknowledged. This material is based upon work supported by NSF (grant nos. DMR-0348585, CCF-0506660 and ECS- 0403489).

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