Highly sensitive integrated pressure sensor with horizontally oriented carbon nanotube network
© Mohammad Haniff et al.; licensee Springer. 2014
Received: 30 December 2013
Accepted: 24 January 2014
Published: 28 January 2014
This paper presents a functionalized, horizontally oriented carbon nanotube network as a sensing element to enhance the sensitivity of a pressure sensor. The synthesis of horizontally oriented nanotubes from the AuFe catalyst and their deposition onto a mechanically flexible substrate via transfer printing are studied. Nanotube formation on thermally oxidized Si (100) substrates via plasma-enhanced chemical vapor deposition controls the nanotube coverage and orientation on the flexible substrate. These nanotubes can be simply transferred to the flexible substrate without changing their physical structure. When tested under a pressure range of 0 to 50 kPa, the performance of the fabricated pressure sensor reaches as high as approximately 1.68%/kPa, which indicates high sensitivity to a small change of pressure. Such sensitivity may be induced by the slight contact in isolated nanotubes. This nanotube formation, in turn, enhances the modification of the contact and tunneling distance of the nanotubes upon the deformation of the network. Therefore, the horizontally oriented carbon nanotube network has great potential as a sensing element for future transparent sensors.
Carbon nanotubes (CNTs) are nanostructured materials used in the production of microelectromechanical sensors because of their outstanding electronic, mechanical, and electromechanical properties [1–3]. CNTs have gauge factors that exceed 2,900, which is an order or a magnitude higher than those of state-of-the-art silicon-based resistors . The excellent strain of CNTs produces a highly piezoresistive network, which benefits pressure sensors and microscale/nanoscale strains with fine resolution. Many studies have examined the fabrication of highly sensitive pressure sensors by depositing piezoresistive CNTs onto the fixed silicon substrate [5–8], in which single-walled and multi-walled carbon nanotubes (SWNTs and MWCNTs, respectively) are utilized as active sensing elements [9, 10].
Recently, flexible electronic devices attract considerable research attention because of their flexibility and transparency . However, the deposition of highly uniform CNTs onto the flexible substrate is hindered by numerous challenges. Two techniques, namely solution deposition and transfer printing method, are proposed for such deposition [12, 13]. Transfer-printed, chemical vapor deposition (CVD)-grown CNTs often outperform solution-deposited CNTs because of their highly aligned formation. Through the CVD method, the size, shape, and area density of CNTs are determined by the chemical composition, plasma, and geometrical features of the catalyst [14–17]. The sensitivity of as-grown CNTs on the application of load is determined by their formation. Therefore, the density and growth formation of as-grown CNTs must be optimized to enhance their pressure sensitivity.
In this paper, the incorporated horizontally oriented MWCNT network on a flexible substrate as a sensing element is presented for the purpose of enhancing sensitivity of pressure sensors in low-pressure applications. The controlled growth formation of this network is determined using an AuFe bilayer as a catalyst. The transfer of resultant nanotubes on the flexible substrate is examined, and their surface morphology and electrical properties are compared to those of the as-grown nanotubes.
Results and discussion
Characteristics of the catalyst nanoparticles and the growth of the resultant nanotubes
Type of catalyst/CNTs
Range of size/diameter (nm)
Growth rate (μm/min)
Connected clusters with small nanoparticles
16.9 to 200
7.0 to 9.0
After applying pressure onto the membrane, the MWCNTs that were stretched via mechanical deformation likely modified the physical structure of the nanotubes in the effective region, which resulted in a loss of contact and an increase in the tunneling distance among the nanotubes as shown in Figure 4b. The contact area and the tunneling distance per nanotube were enhanced during the stretching because of the large portion of isolated nanotubes and the weak van der Waals forces among the nanotubes. Therefore, the electrons that hop between the adjacent nanotubes were reduced, limiting the transport of electrons in a region with lesser contact. Therefore, larger resistance changes resulted from the increased tunneling and contact resistance.
This study used a flexible substrate to incorporate a highly sensitive horizontally oriented MWCNT network in pressure sensing performance. A horizontally oriented MWCNT network with low density was grown on an AuFe catalyst. The nanotube network was successfully transferred from the silicon-based substrate to a flexible substrate with 90% yield rate. Both the as-grown and as-transferred nanotubes were characterized to examine the variations in their morphologies and electrical properties. The fabricated pressure sensor showed a great potential in sensing a small change of pressure with a sensitivity of approximately 1.68%/kPa. A larger portion of isolated nanotubes could enhance the modifications of the contact area and tunneling distance per nanotube, which limited the transport and hopping of electrons due to the loss of contact among the nanotubes. Such modifications eventually increased the resistance changes and pressure sensitivity of the network.
This research was supported by the National Nanotechnology Directorate Funding NND/ND/(2)/TD11-012 and the eScience Funding 01-03-04-SF0027 under the Ministry of Science, Technology, and Innovation (MOSTI), Malaysia as well as the ERGS 203/PMEKANIK/6730069 under the Ministry of Higher Education (MOHE), Malaysia.
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