Sensors and Actuators A 201 (2013) 36–42 Contents lists available at SciVerse ScienceDirect Sensors and Actuators A: Physical jo u r n al homep age: www.elsevier.com/locate/sna Controlling SWCNT assembling density by electrokinetics Haibo Yu a,d , Sascha Hermann c , Zaili Dong a , John Mai b , Wen J. Li a,b,∗ , Stefan E. Schulz c,d a Key State Laboratory of Robotics, Shenyang Institute of Automation, CAS, Shenyang, China b Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Hong Kong, China c Center for Microtechnologies (ZFM), Chemnitz University of Technology, Chemnitz, Germany d Department of Back-end of Line, Fraunhofer Institute for Electronic Nano Systems, Chemnitz, Germany a r t i c l e i n f o Article history: Received 5 December 2012 Received in revised form 13 May 2013 Accepted 13 May 2013 Available online xxx Keywords: Assembly Dielectrophoresis Single-walled carbon nanotubes a b s t r a c t Single-walled carbon nanotubes (SWCNTs) as the colloid in a colloidal solution can be polarized in a non- uniform electric field and experience a net force that is the so-called dielectrophoresis (DEP) force, due to the interaction between the induced dipoles and the electric field. The positive DEP force can be used to position and assemble arrays of SWCNTs. Inversely, the negative DEP force can be utilized to separate SWCNTs in terms of their electronic properties. Moreover, Joule heating generated by the electric field can lead to other electrokinetics forces in the colloidal solution, which give rise to fluidic motion of the solution. Additionally, at low frequencies, the electrical double layer also induces a steady fluidic motion, a phenomenon known as AC electroosmotic flow. These fluidic motion in turn exerts a drag force on the nanotubes. Hence, to controllably assemble SWCNTs using DEP force is a non-trivial task. In this article, the mechanisms of electrokinetics and electrohydrodynamics are systematically analyzed through numer- ical simulations for a set of parameters that are typically used for assembling SWCNTs between metal electrodes. Finally, experimental results from the frequency-dependent assembly of SWCNTs using this set of parameters are described and discussed. These results show that the density of SWCNTs assembled between electrodes can be varied by controlling the electrokinetics parameters. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Applications involving single-walled carbon nanotubes (SWC- NTs) have been one of the most popular research topics in the fields of nanoelectronic devices, nanoactuator systems, and nanosensors, due to their unique geometric structures, excellent mechanical properties, and extremely high electrical and thermal conductiv- ities [1–5]. The SWCNTs are one-dimensional materials with a diameter ranging from sub-nanometer to several nanometers, and their lengths are usually on the order of micrometers [1]. In terms of their electrical properties, the SWCNTs can be divided into two types, namely metallic and semiconducting [1,2]. The remarkable electronic properties of SWCNTs make them one of the most pop- ular candidates as the building blocks of nanoscale devices [3,4]. However due to their nanoscale characteristics and the diversity in electrical properties in SWCNTs, it is more challenging to fab- ricate SWCNT-based nanodevices [5–7]. To date several methods have been developed to integrate SWCNTs into nanodevices such as nanomanipulation using an atomic force microscopy (AFM), con- tact printing and dielectrophoresis (DEP) [8–10]. ∗ Corresponding author at: Department of Mechanical and Biomedical Engineer- ing, City University of Hong Kong, Hong Kong. Tel.: +852 3442 9226. E-mail addresses: wenjli@cityu.edu.hk, wenjungli@gmail.com (W.J. Li). An AFM is one of the most effective tools for observing the topog- raphy of materials and for characterizing the mechanical properties of materials, with a very high resolution on the nanoscale. Imag- ing analysis is based on the interaction between the AFM tip and the sample. The AFM has also shown its capability to manipulate a SWCNT placed on a substrate [11,12]. With the development of robotic-enhanced operation system based on an AFM, the effi- ciency of single SWCNT manipulation has increased. The advantage of AFM-based manipulation is that it gives the operator real-time feedback based on images of both the sample and the substrate. In particular, AFM-based nanomanipulation is very high precision. But the typical AFM cannot simultaneously image and manipulate SWCNTs, which limits its application to rapidly fabricate SWCNT- based nanodevices [13]. Over the last decade, most effort has been addressed toward applying DEP to carry out the separation of nanoparticles, nanoassembly and fabrication of nanosensors [14–17]. DEP has shown its great capabilities for aligning SWCNTs in parallel and assembling them between two electrodes in a liquid medium [7,15]. A SWCNT becomes polarized in the presence of an electric field. The polarized SWCNT is driven by a net force that is known as the DEP force, if the electric field is non-uniform. The non-uniform electric field exists when an alternating current (AC) bias is applied between two electrodes. The SWCNT will be driven by the DEP force toward the gradient of the electric field. If the DEP force is positive, 0924-4247/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2013.05.008