The number and position of assembled nanowires cannot be controlled using most nanowire sensor assembling methods. In this paper, we demonstrate a high-yield, highly flexible platform for nanowire sensor assembly using a combination of optically induced dielectrophoresis (ODEP) and conventional dielectrophoresis (DEP). With the ODEP platform, optical images can be used as virtual electrodes to locally turn on a non-contact DEP force and manipulate a micron- or nano-scale substance suspended in fluid. Nanowires were first moved next to the previously deposited metal electrodes using optical images and, then, were attracted to and arranged in the gap between two electrodes through DEP forces generated by switching on alternating current signals to the metal electrodes. A single nanowire can be assembled within 24 seconds using this approach. In addition, the number of nanowires in a single nanowire sensor can be controlled, and the assembly of a single nanowire on each of the adjacent electrodes can also be achieved. The electrical properties of the assembled nanowires were characterized by IV curve measurement. Additionally, the contact resistance between the nanowires and electrodes and the stickiness between the nanowires and substrates were further investigated in this study.
© 2014 Optical Society of America
Because of the high surface area-to-volume ratio, nanowires are widely used in various fields. For example, they are used in field effect transistors (FETs) to replace the conventional carrier channel in planar FETs [1,2]. In other studies, nanowires are used in solar cells [3,4], fuel cells  and displays . In addition, there are many advantages and potential developmental uses of nanowires in biomedical sensors [7–10]; compared with planar structures, nanowires can provide more surface area for the same volume and, thus, can effectively reflect changes in surface potential when charged ions or molecules attach to the nanowire surface. For example, in a literature review , a bioreceptor-modified nanowire (e.g., modified with antibodies or single-strand DNA) was used to detect protein-protein interactions, DNA hybridization, and protein biomarkers. These biomedical sensors have excellent detection performance, such as the ability to detect very low concentrations and instantaneous signal measurements, indicating that nanowire sensors have advantages such as high sensitivity, real time response and label-free detection.
In general, the fabrication of nanowire sensors can be divided into two categories, top down and bottom up. The former processing route often involves the following steps. First, the needed material is deposited, and then electron-beam lithography is used to define the dimensions of the nanowires; finally, the nanowires are etched. The advantage of this route is the cost saving in mass production because of its compatibility with the semiconductor fabrication process. The drawback of this route is that it might be limited by the materials that can be deposited and the line width of the photolithography and etching techniques; the fabrication equipment has higher requirements [12,13]. The bottom up approach involves first fabricating nanowires by chemical synthesis, such as silicon [14,15], zinc oxide [16,17], copper oxide , and silver nanowires , and then using control technology to assemble the components. Therefore, the fabrication cost of the bottom up approach is lower than that of the top down approach. However, current technology has not yet solved remaining nanowire assembly issues, including how to manipulate the nanowire to move it to the desired position and how to control the number of nanowires assembled.
Methods of manipulating nanowires can be roughly divided into two categories, namely, contact and contactless manipulation. For example, the Langmuir Blodgett method is a contact manipulation method : nanowires are uniaxially aligned at the air-water interface using the Langmuir-Blodgett technique and then transferred in a single step to a planar substrate surface. However, this method can only be applied to hydrophilic substrates and cannot effectively control the number and arrangement direction of nanowires that adhere to the substrate surface. Other studies have used nanowire printing to print nanowires on the substrate ; metal electrodes are then deposited on both ends of the nanowire to fix it, and the electrodes are interconnected with some of the nanowires with random probability. Image analysis software is then used to determine the location of the nanowires in a high-magnification scanning electron microscope (SEM); finally, excess nanowires interconnecting the electrodes are removed using a probe to control the number of nanowires. This method is also a contact assembling method that can control the number of assembled nanowires. However, this method requires an SEM equipped with a precision probe platform to remove the excess nanowires; thus, the equipment requirement is high, and the operation is complex . In contrast, optical tweezers, dielectrophoresis (DEP) force, and optically induced dielectrophoresis (ODEP) force are forms of contactless manipulation. Optical tweezers manipulate objects by focusing a laser light on the object and can precisely manipulate a nanowire to the desired location. However, optical tweezers are prone to causing damage to the object manipulated because of the use of high-intensity laser light and require a sophisticated laser light source system and precision x-y stage [23–25]. Manipulation using DEP force involves inputting alternating current (AC) signals to form a non-uniform electric field on the metal electrode and manipulating the object using the force exerted on the object by the non-uniform electric field. This approach has the advantages of simple operation and an inexpensive, simple experimental set up. However, the controlling range of the DEP force will be limited by the location of the metal electrode deposited, and DEP force cannot precisely control the number of nanowires attracted, i.e., there is little flexibility in the operation [26–28]. The use of ODEP force to manipulate nanowires was first proposed by Jamshidi et al. . In ODEP, dynamically reconfigurable virtual electrodes are generated on a photoconductive surface by projecting a suitable light pattern, and the sample is then manipulated by DEP forces acting in an inhomogeneous electric field; nanowires are manipulated by moving the light image. Jamshidi et al. successfully manipulated metal and semiconductor nanowires using ODEP force in their study. This method is contactless, uses a low-intensity light source, and makes it possible to dynamically manipulate objects by inputting different optical patterns. Thus, this approach has the advantage of great control flexibility. However, using ODEP force alone cannot effectively control the direction of horizontal nanowires after moving them to the target location and thus cannot effectively connect the nanowires to metal electrodes.
In this study, the combination of ODEP and DEP was used to effectively assemble nanowire sensors. An ODEP force platform was used to move the nanowires next to the electrodes Then, AC signals were transferred to the metal electrodes to generate the DEP force, and thus, the nanowires were attracted and aligned to the electrode to complete the interconnection. After connecting the nanowires, platinum is patterned at both ends of the nanowires to clamp the nanowire between the platinum and gold electrode, thus avoiding the problem of moving the nanowires during subsequent processing and reducing the contact resistance and increasing the signal stability. This approach can effectively be used to assemble single nanowires to multiples pairs of electrodes and can continuously assemble multiple nanowires to a single pair of electrodes, thus having the advantage of great operation flexibility and fast assembly.
2. Materials and methods
2.1 Principles of using ODEP force to control nanowires
An optically induced effect allows the generation of a virtual electrode for applying DEP forces in a non-uniform electric field. A schematic diagram is presented in Fig. 1. The AC signal is inputted to the upper indium tin oxide (ITO) glass plate and the conductive ITO layer on the lower photoconductive plate to generate a uniform electric field. Dipoles are induced in the nanowires in this electric field. A light beam is projected onto the amorphous silicon layer to decrease the resistance and thus forms a non-uniform electric field. Then, the nanowire with induced dipoles will be attracted or repelled by the non-uniform electric field. The interaction between the dipole and the non-uniform electric field is the ODEP force, which can be expressed as 
2.2 Strategies to assemble nanowire sensors with high yield
Figure 2 is a schematic diagram of the steps in nanowire sensor assembly. First, the solution containing nanowires was added to the ODEP chip. The nanowires were initially randomly scattered on the substrate, as shown in Fig. 2(a). After inputting the AC signal to the upper and lower plates, the polarized nanowires aligned vertically along the electric field and were randomly distributed in the chip. Then, a commercially available projector, microscope objective, and computer software were used to project a light spot on the amorphous silicon photoconductive layer to generate the ODEP force and attract nanowires, as illustrated in Fig. 2(b). Software (Flash MX 2004, Adobe) was used to move the light source to control the non-uniform electric field and, thus, move the nanowires next to the patterned metal electrodes, as illustrated in Fig. 2(c). If operating with the ODEP force alone, when the electric field was turned off after moving the nanowires next to the electrodes, the nanowires would randomly lie on the substrate in the horizontal direction and could not be aligned across the gap between the two electrodes. Therefore, in the final step, AC signals were transferred to the two electrodes to generate the DEP force. The DEP force attracted the nanowires onto the electrodes and aligned them, as illustrated in Fig. 2(d). In the conventional method, the nanowires must be moved by fluids and only approach the electrodes by random chance when the nanowires are within the range of the DEP force. The presented method can improve the situation encountered in the conventional method of using the DEP force. After connecting the nanowire, once the solution in the chip evaporated, the upper ITO glass plate was removed; then, platinum was patterned at both ends of the nanowires using a focused ion beam (FIB) system to fix the nanowire, as illustrated in Fig. 2(e). After patterning the platinum, the chip was rinsed with acetone and methanol three times to clean the surface and remove the excess nanowires. After cleaning, photolithography was performed to cover the metal electrodes with negative photoresist (SU-8 2005, MicroChem, USA) for insulation, with only the nanowires exposed to serve as the sensing area, as illustrated in Fig. 2(f).
2.3 Photoconductive material and metal electrode fabrication
Amorphous silicon was used as the photoconductive material in this study, and glass was used as the substrate. First, 70 nm of ITO was sputtered onto the glass as the transparent conductive layer. After annealing (240 degrees, 15 minutes), 10 nm of molybdenum (Mo) was sputtered on top to enhance the adhesion between the ITO and amorphous silicon and to reduce the contact resistance. Then, a plasma-enhanced chemical vapor deposition (PECVD) system was used to deposit 1 μm of amorphous silicon as the photoconductive layer. Finally, 100 nm of SiNx was deposited on top of the amorphous silicon using PECVD as an insulating layer to avoid current leakage. Then, the metal electrodes were fabricated using lift-off technology. First, the electrode pattern was defined by photolithography, and then, 30 nm of chromium was deposited as an adhesion layer using thermal evaporation. Later, 120 nm of gold was deposited as the electrode; finally, the photoresist was removed with acetone to remove excess metal. The distance between the two ends of the electrodes was 10 μm. After fabricating the metal electrodes, two pieces of ultra-thin double-sided adhesive (63 μm thick) (8018PT, 3M, USA) were fixed on the substrate, serving to define the height of the fluidic channel, and finally, the upper ITO conductive glass plate was used to cover the top. Then, the nanowire sensors were assembled by following the procedure proposed in 2.2.
2.4 Material preparation and experimental set-up
The silicon nanowires (Sigma-Aldrich Chemie GmbH, USA) used in this study were approximately 20 μm long and 150 nm in diameter. The nanowires were stored in isopropyl alcohol (IPA) solution with an initial concentration of 106 wires per ml to avoid nanowire oxidation. However, we observed that the ODEP force was very small when using IPA as the manipulating solution; the solution evaporated faster, and the nanowires were prone to adhere to the substrate. Therefore, different proportions of IPA were replaced with de-ionized (DI) water (18 MΩ) in the experiment (see section 3.2 for details). Figure 3 illustrates the entire ODEP platform set-up. In this study, a commercially available projector (PLC-XU350, Sanyo, Japan) was used as the light source, with an objective lens (UV Plan 50X, Nikon, Japan) to focus the projector's light, and projected onto the amorphous silicon to change its resistance. The projector was connected to a PC to use computer software to control the position of the projected light spot. Two signal generators (33210A, Agilent, USA) were connected to the ITO conductive layers on the upper and lower plates and to the metal electrodes to generate the ODEP and DEP forces. The imaging system was a microscope tube (Zoom 160, OPTEM, USA) with a 1/2” CCD camera (STC-620PMT, SENTECH, Japan), which can provide approximately 1.1 μm resolution. After connecting the nanowires, 200-nm-thick platinum was patterned at both ends of the nanowires using an FIB (Quanta 3D FEG, FEI, USA). Finally, the IV curve of the nanowire sensors was measured using a semiconductor characterization system (4200-SCS, Keithley, USA) to ensure the quality of the assembly.
2.5 Assessment of the lowest light intensity required to manipulate nanowires
The illumination of photoconductive material creates a virtual electrode, which is then used to generate an inhomogeneous electric field for dielectrophoretic manipulation. It is known that the conductivity of amorphous silicon increases with increasing illumination power intensity . Therefore, in our study, the lowest light intensity required by the system to manipulate nanowires was determined before assembling the nanowire sensors. The light intensity of the light source used in our study – a liquid crystal projector – can reach 3.98 μW μm−2 after focusing by a lens. Because we cannot linearly reduce the light intensity of the projector, neutral density filters (BX3-25ND25 and BX3-25ND6, Olympus, Japan) were used to reduce the light intensity to 25%, 6%, and 1.5% of the maximum light intensity, i.e., to the intensities of 1000 nW μm−2, 240 nW μm−2, and 60 nW μm−2, respectively. The manipulation test confirmed that when the light intensity was attenuated to be 1000 nW μm−2 or 240 nW μm−2, one could still effectively control the nanowires with the ODEP force. However, when the light intensity was attenuated to be approximately 60 nW μm−2, the ODEP force generated was too small to effectively control the nanowires. We estimated that the smallest power intensity that can be used to smoothly manipulate nanowires in the present setup was roughly 200 nW μm−2, which is approximately 104 times (estimated with 1 μm in laser spot diameter and 5 mW in power intensity) less than the light intensity required by optical tweezers to achieve a very similar nanowire manipulation function .
3. Results and discussion
3.1 Assembling nanowire sensors using a combination of ODEP and DEP forces
Figure 4 shows the procedure for assembling nanowire sensors. First, nanowires and the solution were injected into the ODEP chip and allowed to stabilize before the assembling could start, which required approximately 20-30 seconds after the injection, as illustrated in Fig. 4(a). A nanowire to be assembled was selected, and then, the AC signal (frequency = 100 KHz, Vpp = 10 V) was inputted to the ITO conductive layer on the upper and lower plates. Then, the nanowire aligned itself parallel to the electric field (perpendicular to the electrode surface), as observed in Fig. 4(b), and the projector was used to project a light spot (20 μm in diameter) near the nanowire. Under such manipulation conditions, the nanowires were attracted to where the light spot was located by the positive DEP force, as illustrated in Fig. 4(c). The light spot was moved using computer software to guide the nanowire toward the metal electrodes and, eventually, next to the metal electrode, as shown in Fig. 4(d). The ODEP force generated under this operating condition was small (around several pN ) due to the small dimension of the nanowires. The moving speed should not exceed 20 μm s−1; otherwise, the nanowire cannot keep up with the light spot. After moving the nanowire next to the metal electrodes, the AC signal and the light were turned off, as shown in Fig. 4(e). Without the DEP force, the nanowires lay down on the substrate in random lying directions. Note that the distance between the lying nanowires and electrodes should be within the effective range of the AC electric field originating from the metal electrodes. The AC signal was then transferred to the metal electrode (frequency: 100 KHz, voltage: 2 Vpp). At this time, the DEP force generated by the electrode attracted the nanowire to the gap between the two electrodes and finished connecting the nanowire, as observed in Fig. 4(f). In this manner, the nanowires can be connected to different electrodes continuously, and multiple nanowires can be connected to the same pair of electrodes. The time required to assemble a nanowire sensor is related to the initial distance between the nanowire and the electrode. On average, approximately 24 seconds was needed to complete one nanowire interconnect. To avoid the trapping of multiple nanowires, the concentration of nanowires was properly diluted in the working solution before manipulation. However, once trapping of multiple nanowires took place, we sped up the dragging velocity of the nanowires or repeatedly turned on and turned off the applied AC voltage. Some unwanted trapping of multiple nanowires can be addressed this way.
Figure 5(a) presents an example of using this manipulation platform to connect a single nanowire to multiple pairs of adjacent electrodes. The ODEP force and DEP force described above were used in alternation to connect a single nanowire to adjacent electrodes sequentially. The assembling process is depicted in a movie (Media 1). In addition, the same method can also be used to assemble multiple nanowires to the same pair of electrodes. As observed in Fig. 5(b) (Media 2), nanowires scattered in the vicinity of the electrode were pulled, one by one, to the gap between the two electrodes by the light spot and were then attracted to arrange between the two electrodes by the DEP force. Compared with conventional methods, which use a DEP force or fluidic force alone, this method can significantly improve the success rate and flexibility of the assembly and control the number of nanowires to be connected. However, when using the ODEP force to manipulate nanowires, the electrostatic force between the substrate and nanowires can cause the nanowire to stick to the substrate, reducing the amount of nanowires that can be effectively controlled. In our study, this issue was addressed by changing the manipulating solution.
3.2 Effects of the DI water-to-IPA ratio and oxygen plasma treatment of SiNx on nanowire adhesion
In the beginning, the silicon nanowires were stocked in IPA. If the stock solution could be used for the manipulation solution, the sample preparation process would be simple. However, the use of IPA as a manipulation solution presented two problems. One issue was the fast evaporation of the IPA, and the other problem was the small manipulation force due to the small permittivity (polarization) of IPA. In addition, approximately 50% of the silicon nanowires stuck to the substrate. Adhesions caused by the van der Waals force between the nanowires and substrate can cause adverse effects on the manipulation on the substrate. Repulsion between the nanowire surface and substrate surface can alleviate this problem . The silicon nanowires experienced some oxidation so that the surface of the silicon nanowires became SiO2. According to reference , the zeta potential of such silicon nanowires ranges from −27.7 to −41.6 mV. Furthermore, the dominant charge site on SiNx exposed to aqueous solutions is Si-OH, with a minority (around 1%) of Si-NH2 sites . Therefore, the charge polarities of both the silicon nanowires and SiNx substrate surface before oxygen plasma treatment were negative. The electrostatic repulsive force between the nanowires and the substrate arose due to mutual double-layer repulsion from the surface charges. The addition of water increased the mutual repulsive force. The reason for the electrostatic repulsion increase with a high ratio of DI water to IPA is that the zeta potentials of silica and the silicon substance increases as the concentration of DI water is increased in the IPA solution . In this study, the stickiness issue was improved by adjusting the ratio of DI water to IPA in the working solution and by treating the substrate with oxygen plasma. First, the DI water-to-IPA ratio was changed. The proportions of DI water in the solution were 0%, 20%, 50%, 70%, and 100% (volumetric percentage). Multiple nanowires were manipulated with the ODEP force to evaluate the extent of adhesions between the nanowires and the substrate. Figure 6 demonstrates that the adhesion decreased with an increasing proportion of DI water in the IPA solution. The adhesion was the lowest when the solution was pure DI water, in which case approximately 71% (n = 101) of the nanowires did not adhere to the substrate before oxygen plasma treatment of the substrate. This is a large improvement over the original ratio, for which 49% (n = 23) of the silicon nanowires did not adhere to the substrate in pure IPA solution; effective manipulation using the ODEP force can thus be implemented. Next, to further improve the adhesion situation, surface modification was performed on the substrate with oxygen plasma at 30 W for 2 minutes. After the SiNx surface was exposed to oxygen plasma, the surface became hydrophilic and had a larger zeta potential than the unexposed surface. It was expected that the zeta potential of the surface would be similar to that of the silicon nanowires with oxidated surfaces and silicon dioxide (approximately −40 mV) . Therefore, oxygen plasma treatment can further improve the electrostatic repulsion. The circular marks in Fig. 6 indicate that at a DI water proportion of 100%, the proportion of free nanowires improved from the previous 71% to 80% (n = 100) for the oxygen plasma-treated substrate. On the other hand, during the nanowire manipulation, the nanowires were immersed in the solution, such that no significant surface tension forces influenced the manipulation when the nanowires were moved.
3.3 I-V properties of nanowires with platinum patterned at both ends
Although the nanowires were successfully situated on top of the electrodes with the aid of ODEP, the currents passing to each nanowire had large variation due to poor electrical contact between the nanowires and electrode. In addition, the photolithography in the subsequent processing, which is needed for electrical insulation, may remove the connected nanowires. Therefore, after connecting the nanowires and evaporating the solution, platinum was patterned at both ends of the nanowire using an FIB system, such that the nanowire was clamped between the platinum and gold electrode. It was observed that an excessively high ion beam power could damage or even cut off the nanowire, resulting in an open circuit. Therefore, an ion beam with a voltage of 15 kV and a current of 30 pA was used to pattern the 200-nm-thick platinum at both ends of the nanowire. Figure 7(a) shows a scanning electron microscope (SEM) image of a nanowire sensor with patterned platinum. Depositing platinum can enhance the quality and stability of nanowire interconnects. To obtain quantitative data, the IV characteristics of the nanowire sensors were measured with a Keithley 4200-SCS. Figure 7(b) and Fig. 7(c) present the IV characteristic curves before and after depositing platinum at both ends of the nanowires, respectively. The experimental data show that in addition to fixing the nanowires to metal electrodes, depositing platinum can also effectively improve the contact interface between the nanowire and metal electrode, resulting in more stable current conduction and more consistent assembly quality. The coefficient of conductance variation improved from 82% (n = 8) without deposited platinum to 12% (n = 5) with deposited platinum, and the contact resistance was significantly reduced. The average conductance increased from 3.26 pS before patterning the platinum to 59.88 pS after patterning the platinum. In addition, the lifetime of the assembled nanowire sensors – the number of days that the sensor can operate – was also assessed in our study. As shown in Fig. 7(d), the electrical properties were measured every 2 days from day 2 to day 28. The coefficient of conductance variation was approximately 10.82% for the 14 measurements, and the electrical signal can still be measured after 28 days (without SU-8 coating). In addition, the metal electrodes were covered using negative photoresist (SU-8 2005), and only the nanowires were exposed, such that the sensor could perform measurements in a wet environment. Figure 8 shows changes in the current of the nanowire when performing measurements in solutions with pH values of 6, 8, and 10. The figure indicates that the current passing through the nanowire decreased with increasing pH values of the solution. This result occurs because the hydrogen ions in the solution were adsorbed to the surface of the nanowires and thus changed the surface potential of the nanowires and further changed the conductance of the nanowires. The effect of hydrogen ion adsorption on the silica or silicon surface has been described by the site-binding model . Surface terminating –SiOH groups were formed on the silicon nanowires, which were then deprotonated to SiO-. Positively charged ions, such as hydrogen ions (H+), were then adsorbed on such charged sites to form surface complexes, which influenced the conductance of the silicon nanowires. The density of –SiOH groups on the nanowire surface determines its sensitivity to the hydrogen ions. Note that the pH measurement demonstrated in Fig. 8 was just a proof of concept that the assembled nanowire device could be used in a wet environment. Therefore, a complete verification of the stability and reproducibility of the wet environment measurement was not performed in this study. However, the decreasing trend of the current magnitude with increasing pH was ensured by several measurements. Our focus was to demonstrate a working device fabricated by the controlled assembly of nanowires using an ODEP platform; the measurements in a wet environment are still ongoing.
In this study, nanowire sensors were successfully assembled by the combination of ODEP and conventional DEP forces. This approach greatly increased the success rate of assembly compared with the conventional method of using only the DEP and fluidic drag forces and can control the number of nanowires to be assembled. In addition, our results indicate that the issue of nanowire adherence to the substrate can be effectively improved with a manipulating solution of pure DI water on an oxygen plasma-treated SiNx substrate. The proportion of nanowires that did not adhere and could thus be manipulated by the ODEP force was greater than 80%. In addition, the measurements also indicate that effective manipulation could still be performed at light intensities as low as approximately 200 nW μm−2, which is 30 times less than the light intensity required by optical tweezers. Depositing platinum at both ends of the nanowires after interconnecting improved the current signals and stabilized the signals of the nanowire sensors. Sensors with appropriate insulation were also successfully used for detection in wet environments. The method proposed in this study can be used to simultaneously assemble multiple pairs of electrodes, control the number of nanowires to be assembled, and provide an accurate and versatile nanowire sensor assembling platform.
The authors would like to thank the National Science Council of Taiwan, Industrial Technology Research Institute, and Chang Gung University for financial support under Grant No. NSC 100-2221-E-182-021-MY3, GERPD2B0021, and UERPD2B0091.
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