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Single carbon nanotube infrared detectors

A newly developed nanorobotic manipulation system allows the fabrication of an IR detector with high sensitivity and low noise.
1 February 2007, SPIE Newsroom. DOI: 10.1117/2.1200701.0514

Due to their unique hollow cylindrical structure, carbon nanotubes (CNTs) are considered very promising for many potential nano-electronics and nano-device applications. Their optical properties have been extensively explored by absorption spectroscopy1 and Raman scattering,2 and their photoconductivity under infrared illumination has also attracted considerable interest since CNTs can potentially be used as superb infrared detector materials characterized by high sensitivity and low noise.

Single-wall carbon nanotube (SWNT) films are now being used for photodetection. However, since it is difficult to fabricate CNT films in which all nanotubes have similar properties, they are typically plagued by poor performance. Theoretical studies have shown that the photoconductivity of individual SWNTs perform much better than CNT films, but the fabrication process of SWNT-based detectors is usually complicated and inefficient. As a result, the R&D scope of single CNT-based IR detectors has been significantly limited.

However, advances in atomic force microscopy (AFM)-based nanorobotic manipulation systems3,4 have allowed the recent development of a new deterministic process that can reliably and efficiently fabricate single CNT-based nano-infrared detectors.5

Detector design and fabrication
The design of an individual SWNT-based IR detector is illustrated in Figure 1. It consists of a pair of electrodes bridged by a single nanotube. The fabrication process starts with the electrodes, made using an optical lithography method. Next, the substrate is coated with a drop of SWNT suspension and an AC signal is applied between the two electrodes. Several SWNTs are usually trapped around the electrodes by the dielectrophoresis force acting on neutral bodies in nonuniform electric fields. Finally, the AFM-based nanorobotic manipulation system is used to position one CNT as a connector between the electrodes and to clean out the other CNTs. Figure 2 shows the AFM image of a SWNT-based IR detector.

Figure 1. Diagram of an individual SWNT-based IR detector.

Figure 2. An AFM image of an individual SWNT-based IR detector. Inset: AFM image of the area between the electrodes.
Experimental results
Since a SWNT has different functionality with gold electrodes, a Schottky contact is formed at each of its ends. This allows the device to function like two reversely connected Schottky diodes. Figure 3 shows the measured I-V curve of an individual SWNT-based IR detector, which is clearly exhibiting Shottky diode behavior.

Figure 3. The I-V curve of an individual SWNT-based IR detector.
The photocurrent response under multiple on/off IR illumination cycles is shown in Figure 4. In these experiments, carried out at room temperature, the incident IR power was set at 30mW. A constant bias voltage of 50mV was also applied across the SWNT and the current was monitored. The plot shows that the current immediately increases at the onset of IR irradiation, falling back to its original value when the irradiation is switched off. Tests were also conducted to measure the dark current of the detector at different temperatures, with typical results shown in Figure 5. At the same temperature, much smaller dark currents were observed relative to other IR detector materials (data not shown).

Figure 4. The Temporal photocurrent response of an individual SWNT- based IR detector.

Figure 5. The I-V curve of an individual CNT-based IR detector at different temperatures.


The newly developed AFM nanorobotic manipulation system provides an efficient tool to fabricate SWNT-based nano IR detectors. It also has the potential of being applied to the manufacture of other nano devices and systems. Our SWNT-based nano IR detectors have excellent sensitivity, response time, and dark current characteristics. Future studies will further demonstrate that the efficient use of the nano properties of CNTs can produce IR detectors with unprecedented performance.

Jiangbo Zhang, Ning Xi, King Lai
Michigan State University
Electrical and Computer Engineering, USA

Jiangbo Zhang received his BS from Tsinghua University in 2000, and his MS from the Shenyang Institute of Automation, Chinese Academy of Sciences, in 2003. Currently, he is working on a PhD at the Department of Electrical and Computer Engineering at Michigan State University. His research interests include micro/nanorobotics and systems, nanoelectronics, and the design and fabrication of CNT- based nanoelectronic devices.

Ning Xi received his DSc from Washington University in 1993. Currently, he is John D. Ryder Professor of Electrical and Computer Engineering and the director of the Robotics and Automation Laboratory at Michigan State University. His research interests include robotics, manufacturing automation, micro/nano systems, and intelligent control and systems.

King Wai Chiu Lai received his PhD in Automation and Computer-Aided Engineering at the Chinese University of Hong Kong in 2005. His research interests are micro-assembly, fabrication of bio-MEMS devices, and CNT manipulation and classification. He received the best conference paper award at the International Conference on Intelligent Mechatronics and Automation (ICIMA) in 2004.

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