From Single-Nanowire Biosensors to Nanowire Networks for Transparent Electrodes: A Framework to Reduce Fabrication Cost and Improve Device Functionality

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ScienceDirect Materials Today: Proceedings 19 (2019) 3–14

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NANOTEXNOLOGY2018

From Single-Nanowire Biosensors to Nanowire Networks for Transparent Electrodes: A Framework to Reduce Fabrication Cost and Improve Device Functionality Mahshid Sama*, Rustom B Bhiladvalaa a

Department of Mechanical Engineering, CAMTEC –Center for Advanced Materials & Reated Technologies and IESVic –Integrated Energy Systems Victoria, University of Victoria, Victoria, BC, V8P 5C2, Canada

Abstract

Field-assisted nanowire positioning is a cost-effective technique that uses an AC electric field for fabricating nanowire devices. The values of several variables involved, such as the electrode geometry and the frequency and magnitude of the applied field, need to be chosen to increase overall area coverage, while maintaining functionality. Insufficient understanding of the effect of each variable can escalate disruptive forces and effects, e.g. electroosmotic flow and electrode polarization, that hinder nanowire positioning. We introduce a quantitative guideline for calculating variable values to fabricate nanowire devices, which replaces a tedious and costly trial-anderror process. Earlier experimental demonstrations of attogram-level mass sensitivity with nanoresonators were made using nanopatterning methods, cost-prohibitive for commercial application. We demonstrate a low-cost method for making biosensors of the same sensitivity, with high yield, and for large transparent electrodes with high transmissivity and conductivity. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 11th International Symposium on Flexible Organic Electronics (ISFOE18). Keywords: Nanowire; nanosensor; transparent electrode; field-assisted nanowire positioning

* Corresponding author. Tel.: +1-250-634-3422; fax: +1-250-721-6323. E-mail address: [email protected] ; [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 11th International Symposium on Flexible Organic Electronics (ISFOE18).

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Nomenclature NW DEP pDEP EO EP SEM

1.

nanowire dielectrophoretic positive dielectrophoretic electroosmosis electrode polarization scanning electron microscope

Introduction

Nanowires (NWs) have been used in many optoelectronic and sensing devices [1–4] due to their distinct properties such as high surface to volume ratio, low mass, high crystallinity and directional electron transport. We can classify NW devices into two groups: 1- devices in which single NWs are positioned in predetermined places [1,4]. The most common nanodevices of this kind are used for generating detection signals such as mass or gas sensor. 2- devices in which a large number of NWs are connected to generate large-size optoelectronic devices. Uniform networks of NWs enable high electrical conductivity and optical transmissivity required for applications such as photovoltaics and wearable electronics [2,3]. Precisely controlled assembly, uniform dispersion, long-range order, and good connection at NW junctions, are some of the requirements for fabricating high-performance devices from NWs. Developing a technique capable of fabricating high device yield at low cost is challenging. Two basic families of fabrication techniques are termed top-down and bottom-up [5]. The top-down approach starts with patterning of structures, typically from thin films. The most common top-down approaches involve optical or X-ray lithography, e-beam and ion-beam lithography and imprinting. In this technique parts are both patterned and built in place, and therefore no assembly step is needed. It is easy to create well-ordered structures, with a high homogeneity in size and uniform areal density. Particularly for nanostructures to be made smaller than the wavelength of light sources available for patterning, top-down methods involve patterning that requires the rastering of an electron beam, for which the fabrication time and cost scale with device area. This makes the cost prohibitive for most large-area applications. Furthermore, deposition and patterning steps can produce nanostructures with high distortion or residual stress, particularly severe for metallic structures. In addition, fabricating suspended structures with cross-sections other than rectangles is not feasible. In the bottom-up approach, individual atoms and molecules build up the desired nanoscale elements, in some cases through inventive use of self-assembly [5]. Some examples of techniques for the synthesis of nanoscale elements are vapor-liquid-solid (VLS) growth, chemical vapor deposition (CVD), sol-gel processing and layer-bylayer self-assembly. The bottom-up technique can be used to generate structures with any geometry and shape. The nanostructures can be assembled on any substrate, not limited to those in use for standard CMOS processes. In the bottom-up approach, structures of different materials or with different surface functionalization can be made on one substrate, enabling multiple functions for different elements on the same device, or multiplexing. A biosensing device that can detect different biomarkers on the same chip, provides an example of multiplexing. However, limited control of the ordering process in bottom-up self-assembly is normally a significant drawback. Positioning of assembled patterns on specific locations has proved difficult in fabricating the required structures in practical applications. So, which approach is better? The top-down technique is more suitable for producing structures with long-range order and for making macroscopic connections, but residual stresses, unwanted deformation or processes that are slow or expensive motivated researchers to look for alternatives. In the last two decades several groups have combined bottom-up and top-down techniques to benefit from precision in latter and low fabrication cost in the former approach [1,6–9].

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External forces such as fluid shear force [8,9], magnetic [6] and electric fields [1,8,10], and mechanical forces [7] have been used to assemble the nanostructures on a patterned substrate. Among these forces and methods, the use of AC electric fields has shown great capabilities in positioning single NWs, and more recently in the creation of connected chains of NWs [11,12]. In AC electric field assembly of NWs, the electric field distribution was first created on a substrate through patterned assembly electrodes. This was followed by the introduction of NWs in a liquid suspension on the substrate [1,11–13]. When the gap size between the assembly electrodes is of the order of the average length of NWs, single NWs are drawn into the gap between a pair of electrodes [1,8,13]. NW positioning occurs as a result of the dielectrophoretic (DEP) force [14] exerted on the polarizable nanostructures suspended in a medium in the presence of a non-uniform electric field. In the case where the gap size is chosen to be several times larger than the average length of NWs, dipole-dipole interaction between the ends of two NWs draws them into chains and the DEP force at the edge of the assembly electrodes pulls the NW chains towards the electrodes [11,12]. The chains are assembled parallel to the field lines, in the gap between assembly electrodes. Thus, using AC electric fields, we can control the placement of single NWs, or the connectivity and orientation of chained NWs. These capabilities are essential for fabrication of high performance devices with high throughput. There are difficulties in positioning single NWs or in covering large surface area with multiple connected chains of NWs by using field directed assembly. Applying an AC electric field to a NW suspension can trigger unwanted electrohydrodynamic effects such as electroosmotic flow and electrode polarization, which can disrupt NW positioning. The choice of values of physical parameters determine the strength of these disruptive effects as well as the directive effects (DEP or dipole-dipole interaction). This fact is often overlooked, leading to a time-consuming process for determining suitable parameter values. Examples of these parameters are the electrical properties of NWs and the medium, geometry of electrodes and the magnitude and frequency of the applied voltage [13,15]. The imperfect understanding of possible coupling between these physical effects can prevent reproducibility of published results by a new user utilizing different values of these parameters. Usually the type of the material and geometry of the assembly electrodes are determined by the functional requirements of the device. Given these, we must determine the range of frequency in which the effect of director forces dominates over disruptor forces in the assembly process. In the absence of a framework to guide the choice of parameter values, considerable trial-and-error is needed to obtain the best yield from NW assembly. Here, we present a general guideline based on a physical and mathematical analysis, which can be used to find suitable values of positioning parameters. Field-assisted NW positioning can be used to generate a high yield of single NW devices as well as large rigid or flexible substrates covered with multiple long chains of NWs. Industries such as healthcare, wearable electronics, displays and solar cells can benefit from this nanostructured fabrication approach. 2.

Materials and Methods

We used Ag NWs for creating multiple long chains of NWs for fabricating transparent electrodes. Ag NWs with average length and diameter of 35 µm and 115 nm, respectively, were bought from Sigma Aldrich. Ag NWs were suspended in 2-propanol with electrical conductance of 6×10-6 S m-1 and the concentration of the suspension was 3.9 mg/ml. Different concentrations of NWs, from 0.08 mg/ml to 0.8 mg/ml, were produced by diluting the NW suspension in 2-propanol and used to generate transparent electrodes with different light transmissivity and sheet resistance. Nanoresonators were fabricated using Rh NWs. We used electrodeposition to grow the Rh NWs within porous aluminum oxide templates (Whatman) at -400mV with respect to a Ag/AgCl reference electrode, from an aqueous rhodium sulfate solution (RH221D from Technic) with 60 mM concentration of elemental rhodium. To extract the NWs, the template was soaked in 3M NaOH and sonicated for 15 min to dissolve the aluminum oxide membrane. Subsequently, the suspension was centrifuged and supernatant NaOH solution discarded. NWs were then rinsed with DI water and ethanol and finally suspended in ethanol [16] with electrical conductance of 2.2×10-5 S m-1.

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Figure 1. (a) Single NWs (seen within dashed green boxes) are placed inside wells (red arrows) patterned between an electrode pair with a gap size shown by white arrows. (b) AC electric field applied to assembly electrodes for NW chaining and transparent electrode fabrication with electrode gap sizes shown by green arrows.

We used conventional microfabrication to pattern assembly electrodes on a rigid substrate (glass or Si substrate with 300 µm SiO2 layer) by using photolithography and lift-off process. 50 nm of Au film was deposited on a photolithographically patterned glass substrate. A thin film of Ti (5nm) was deposited as an adhesion layer between Au film and the glass substrate. For single NW assembly, an AC electric field with voltage of 7 V (rms) at a frequency of 100 kHz was applied to the electrodes. The gap size between electrodes for single-NW assembly was 10 µm. To control the placement of single NWs, a photoresist layer was spun on the electrodes and wells (shown in Figure 1 (a)) were photolithographically patterned in photoresist between each electrode pair. An AC electric field was then applied, followed by introduction of NW suspension, which gives rise to the dielectrophoretic force directing NWs into wells. Photoresist was then removed from one of the electrode pairs and metal was electrodeposited to clamp one end of the NWs [13]. Several optical and SEM images of nanoresonator sites were used to calculate the device yield. For chaining NWs, the gap between electrodes was made much larger than the NW length, with gap values of 120, 180 and 240 µm as shown in Figure 1 (b). An AC voltage, with 5V (rms) amplitude and 10 kHz was used for generating chains of NWs. Our guideline to choose these and other values of positioning parameters is explained in following sections. NW suspension was introduced over the substrate after applying the electric field. This technique can be used for fabricating rigid and flexible transparent electrodes (TEs). Rigid TEs were created with NW chains on a glass substrate, with the chains making direct contact with the assembly electrodes. Flexible electrodes were made on a polymer substrate. To do so, the assembly electrodes were first coated with a polymeric film. An AC electric field was applied to the electrodes while NW suspension was introduced over the polymeric film to create the NW chains. The film decorated with chained NWs can be then removed and attached to a desired substrate. Since in transparent electrodes, the entire substrate needs to be covered with NWs, there is no need to precisely control the location of NWs. However, NW connectivity must be controlled by choosing suitable values for assembly parameters such as magnitude and frequency of the applied field, electrode gap size and the thickness of the polymeric film. Light transmission of the substrates was measured using a UV-Vis spectrometer (Cary 50). A blank glass substrate was used as a reference for light transmissivity measurement. Sheet resistance of the NW chains was measured using a Keithley 6340 source meter. 3.

Results and discussions

3.1. Introducing disruptors and directors in field-assisted positioning process Here we introduce the director forces for single and chained NW assembly and the disruptors that are strongly coupled with the director forces. Other disruptors that are not strongly coupled with the director forces have been

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introduced in our previous publication [13]. The weakly-coupled disruptors are not explained here not because of their lack of importance but because they are more application dependent. The director force for positioning single NWs is positive dielectrophoretic force (pDEP). The chaining of nanowires requires, in addition, dipole-dipole interaction as a director force. In field-assisted positioning, negative dielectrophoretic force (nDEP), electroosmotic (EO) force and electrode polarization (EP) are disruptors strongly coupled with the director forces. 3.1.1. Negative dielectrophoretic force The time averaged DEP force is given by [14,17],

1 FDEP  .  m Re( FCM ) . E 2 2

(1)

 is a geometry factor,  m is the electrical permittivity of the medium, and Re( FCM ) is the real part of Clausius-Mossotti factor.  and FCM express differently for cylindrical and spherical shapes: where

FCM  Spherical 

 *p   m*  m*

(2)

Spherical  2r 3 FCM Cylindrical

(3)

 *p   m*  *  p  2 m*

Cylindrica l 

r 2 l

*   i

(5)

2

where r and l are radius and length of the particles, respectively. particle and medium, respectively, and defined as,

(4)

 *p and  m* are

the complex permittivity of the

 

(6)

where  is the electrical conductivity,  is the angular frequency of the applied field and i   1 . nanostructures can be attracted to or repelled from the assembly electrodes when DEP is positive or negative, respectively. In case of spherical particles, nDEP can occur at frequencies as low as 105 Hz while for NWs, nDEP occurs at frequencies higher than 1010 Hz. In this paper, since we are focused on using NWs, we eliminate the effect of nDEP by choosing frequencies below 1010 Hz. 3.1.2. Electroosmotic force Applying an AC electric field to a medium can cause a vortical flow known as electroosmotic (EO) flow as a result of an interaction between electric field and accumulated charges at the surface of a substrate/electrode [13,15]. EO flow is driven by F  qEt , where q is the surface charge density and Et is the tangential component of the applied electric field. The velocity associated with EO vertical flow is frequency dependent as defined here:

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

2  mVrms 2  4 x (1  2 ) 2

(7)

 is dynamic viscosity of the medium and x is the half of the gap between electrodes.  is dimensionless frequency defined as:



 2

x (

m ) m

(8)

where  is the reciprocal Debye length of the double layer. For liquids with a low dielectric constant, such as ethanol,  -1 is ∼0.5 μm. When the EO velocity is high, the EO vertices drag NWs away from the gap between electrodes and disrupt positioning the NWs inside the gaps. Thus, low EO velocity is required for field-assisted positioning of NWs. Maximum EO velocity occurs when  =1 and therefore  max 

2  mVrms . Normalized electroosmotic velocity N EO 16 x

is defined as:

N EO 

 42  max (1  2 ) 2

(9)

At low frequencies, the electrical double layer above electrodes creates a shield for electric field and the voltage drops across the thickness of the electrical double layer. At frequencies higher than relaxation frequency of ions and polar molecules inside the medium, the double layer does not have sufficient time to form and therefore charge accumulation on the electrodes is small, which leads to small EO force. Thus, at very low and very high frequencies, EO velocity is negligible due to small Et and q , respectively. Frequencies at which EO velocity is negligible are suitable for field-assisted NW positioning. 3.1.3. Electrode polarization effect Electrode polarization (EP) occurs as a result of charge accumulation on the electrode surface and causes a voltage drop across the electrical double layer [13]. As a result, the effective voltage (Veff) for positioning NWs is lower than the applied voltage (Vrms). EP can result in small effective voltage and weak pDEP force and disrupt NW positioning. Considering two electrodes separated from each other by an arbitrary gap size x , the electrical conduction path consists of medium with resistance Rm for conduction through the suspending medium, with two capacitive impedances (Cd) in series, associated with the electrical double layers at the two electrodes. The total impedance ZT is defined by [13]:

Z T  Rs [1  The ratio (

Veff Vrms

2 ] j  C d Rm

), the normalized effective voltage for DEP, is defined by:

(10)

M. Sama and R.B. Bhiladvala / Materials Today: Proceedings 19 (2019) 3–14

NV 

Veff Vrms



1 2 1 jRmCd

1

 1 (

9

(11)

2

RmCd

)2

The EP effect is also frequency dependent. Frequencies at which the double layer is very thin and the effective voltage is very close to the applied voltage are suitable for field-assisted positioning of NWs (NV is close to 1 and NEP is close to zero). Normalized electrode polarization can be defined as N EP  1  N V . As discussed above, the presence and the competition between director and disruptor forces depend on the system geometry, electrical properties of materials, and voltage and frequency of the applied field. In the next section we investigate the effect of frequency on field-assisted positioning of single and chained NWs. The type of materials and the geometry of the electrodes are explained in the Materials and Methods section.

Figure 2. (a) the magnitude of the DEP director as a result of the change in the magnitude of Re [FCM] [13]; (b) and (c) show the change in disruptors by frequency for single NW and chained NWs, respectively. Three regions of director-dominant (green), disruptor-dominant (pink) and competing region (yellow) are shown.

3.2. Effect of applied frequency on assembly of NWs by using field-assisted positioning process Here we compare the change in Re [FCM] that represent the director DEP force (Figure 2(a)) with the change of NEP and NEO by frequency. The suitable field-assisted positioning frequency is the one at which NEO and NEP are minimum (NV is maximum) and Re [FCM] has the maximum possible value. We name that frequency the cut-off frequency (Fcut-off). The effect of disruptors is weak at frequencies equal and above the cut-off frequency. We calculated the cut-off frequency for two systems: 1- single NW positioning between electrodes with 10 µm gap size (Figure 2 (b)), 2: NW chain assembly between electrodes with 240 µm gap size (Figure 2 (c)). Graphs (b) and (c) are divided into three regions each represented with a different color:  Disruptor-dominant region (pink) shows the frequencies at which disruptor forces are strong (high NEO and NEP and low NV). At this region the voltage available for positioning is very small and high velocity of electroosmotic flow disrupts the positioning process.  Competing region (yellow) shows frequencies at which NEO is decreasing and NV is increasing. smaller velocity of electroosmosis flow and stronger available voltage can improve the positioning process, but the disruptors are not negligible and compete with the director forces. Director-dominant region (green) shows frequencies at which the effect of disruptors is minimum. The velocity of electroosmosis is negligible and effective voltage is close to the applied voltage (NV =1). Choosing frequencies at

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this region can result in high yield NW positioning. But does increasing the frequency mean a higher positioning yield? Although at frequencies higher than Fcut-off the effect of disruptors is minimum, increasing the frequency reduces the director DEP force as shown in Figure 2(a). Therefore, it is important to find the cut-off frequency for every system to benefit from the maximum director force and minimum disruption of disruptor forces and effects. Finding the cut-off frequency, we can increase the DEP force by increasing the applied voltage. In next section, we show examples of field-assisted assembly for single and chained NWs, using Fcut-off introduced here. 3.3. Fabrication of nanoresonators with high yield Nanowires with small mass (M), and a high mechanical resonance peak frequency (f ) show a decrease in frequency (f ) which is sensitive to added mass (M). The simplest one degree of freedom representation yields the commonly used relation M = (2 f M) / f, so that a nanowire cantilever with typical values of picogram (10-12 g) level mass, 107 Hz frequency and 10 Hz frequency shift upon mass addition, enables attogram (10-18g) level mass detection. The first experimental demonstration of the measurement of attogram level mass with nanocantilevers of mass and stiffness comparable to our nanowires, are over a decade old [18]. Several promising applications of these nanoresonators, such as molecular diagnosis for early cancer detection, have not been realized commercially, because the only known nanopatterning methods used to make them cannot be scaled up to make clinically useful numbers of devices at a reasonable cost. Nanoresonators were used for a first demonstration of prostate cancer biomarker molecule detection [19]. In that work, NWs were functionalized off-chip to bind specific prostate cancer biomarker molecules, before assembly and clamping on a substrate. The systematic use of a physical and mathematical framework to guide the selection of assembly parameter values was not attempted in that work. It is the focus of the single-NW device assembly in this paper. We used field-assisted NW assembly to position Rh NWs in predetermined locations on a lithographically patterned substrate (see Materials and Methods). We used 104 and 105 Hz for NW positioning and we observed an increase in NW positioning yield, from 20% to 92% respectively at 7 V (rms) [13]. The increase in the NW positioning yield is in accord with the theoretically defined cut-off frequency, 105 Hz, for single NW positioning. To measure the yield, we used two substrates on each 9500 sites were available for single NW positioning, as shown in Figure 3(a). Several optical micrographs such as the one in Figure 3(b), were used to determine the yield. The field-directed assembly typically requires clamping of the NWs, at one or both ends, to secure them in place for a circuit or network (integration), or to build individually addressable NW devices. Clamping methods such as electron beam induced deposition (EBID) [10] incur a cost proportional to the number of clamped devices and the cost is prohibitive for large arrays. We used a cost-effective electrodeposition technique to clamp one end of positioned NWs with silver [13]. As shown in Figure 3 (c, d, e) a firm and uniform interfacial contact is generated between Ag clamp and NWs. 3.4. Fabrication of transparent electrodes from multiple long chains of NWs We previously showed that by increasing the electrode gap size beyond the average length of NWs, dipole-dipole interaction connects the polarized NWs while pDEP place the chained NWs on the assembly electrode sites [12]. In our experiments, the assembly electrodes were patterned on a glass substrate by using photolithography and a lift-off process. The details of substrate fabrication are provided in the Materials and Methods section. We used 104 Hz cut-off frequency for NW chaining process. Generating multiple chains of NWs and nanoparticles using fielddirected assembly can be challenging as the very first chain that bridges the electrodes can create a short circuit and prevent the growth of other chains. This effect can be prevented by controlling the applied voltage. In the suspension, a cloud of counter ions adjacent to the substrate and around each pole of the NWs, creates an electrostatic force, which repels NWs from the electrodes. A balance between electrostatic repulsion force and DEP

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attraction force would make NW chains hover above electrodes. We observed this behaviour at 5 V (rms) where the voltage creates strong enough filed to generate chains but the pDEP is in balance with repulsive electrostatic force at the edge of the assembly electrodes. As a result, multiple chains of NWs can be generated, hovering above electrodes.

Figure 3. (a) A photolithographically patterned substrate for single NW positioning; (b) Single NWs positioned inside the gaps between assembly electrodes; (c) SEM image of two nanoresonators. (d) and (e) show single NW resonators clamed from one end [13].

These chains will contact the substrate after the suspension dries out [12]. Figure 4(a) and (b) shows multiple chains of NWs on a glass substrate with 120 and 240 µm electrode gap sizes, respectively. The gold assembly electrodes provide conductivity in the horizontal directions while NW chains create a conductive path in vertical direction. We used different electrode gap sizes (120, 180, and 240 µm) for NW chaining by using 0.25 mg/ml of NW suspension. We measured the light transmission and sheet resistance of the substrates. To improve the light transmission, we reduced the thickness of assembly electrodes from 50 nm to 15 nm which increased the sheet resistance of Au film from 2.5 to 11.5 Ω/sq. The sheet resistance of chained NWs was measured as 30 (Ω/sq). The change in the light transmission of the samples by changing the thickness of the assembly electrodes is shown in Table 1. Due to high demand on flexible and light weight devices such as organic solar cells or bendable screens, we replaced the glass substrate with a thin polymeric film (Polyimide 1388 VTEC) with 8 to 10 µm thickness. Polyimide was coated on a rigid substrate patterned with assembly electrodes. To generate flexible transparent and conductive films using chained NWs, the AC electric field is applied to the electrodes under the polymeric film while NW suspension is introduced on top of the film. After the NW chaining process is completed, the film decorated with NW chains can be peeled and attached to a different substrate. Figure 5(a) shows SEM image of NW chains on polyimide. In Figure 5(b) a flexible transparent electrode made of chained NWs is peeled from the rigid substrate and attached to a plastic film. Field-assisted assembly of chained NWs on polymeric substrate enable stacking different layers on top of each other to generate conductance in selected directions. For example, a layer with NWs chained in vertical direction can be placed above a layer on which NWs are chained in horizontal direction.

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Figure 4: NW chaining on glass substrates with (a) 120 µm and (b) 240 µm gap sizes. Assembly electrodes are shown by red arrows in (a). Table 1. Light transmission of substrates with three different gap sizes for two different thickness of the assembly electrodes, 50 and 15 nm Gap size Thickness of electrodes Light transmission (µm) (nm) % 120

15

87.5

50

65

180

15

90

50

69

240

15

92

50

72

Field-assisted NW positioning enables control over placement, direction and, if required, connectivity of NWs. The direction of NWs is parallel to the electric field lines. DEP force directs the positioning and NWs are placed where the DEP force is maximum, e.g. inside the wells for single NW positioning [13] or at the edge of the assembly electrodes for NW chaining in which, dipole-dipole interaction directs the NW connectivity [12]. In single NW resonators, photolithographically patterned wells generated on the photoresist layer above the electrodes, create areas with stronger electric field gradient and DEP force to trap the NWs (see Figure 3(a) and (b)). In transparent electrodes however, that precision in NW placement is unnecessary as connected NWs must cover the entire surface of the substrate. If for a certain use or application, long and straight lines of connected NWs are required, long photolithographically patterned wells can be generated to control the accuracy of the placement of NWs. Another suggestion is using surface functionalized substrate and NWs so that NW chains are only attached to predetermined places on the substrate. It must be noted that un-curved NWs must be available for creation of straight lines of chained NWs. High aspect ratio NWs, same as Ag NWs we purchased from Sigma Aldrich, have curved shapes.

Figure 5. (a) SEM image of Ag NWs chained on a polyimide substrate; (b) the polyimide substrate with chained NWs is peeled from the rigid substrate and attached to a plastic. Red dashed line shows the area that is covered with NW chains.

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Here, we introduced electroosmotic force and electrode polarization effects as two most important disruptor forces that can interrupt field-assisted NW positioning process. Several assembly parameters such as NW suspension, electrode gap sizes, and the frequency of the applied AC field affect the strength of director and disruptor forces and effects. To obtain a better understanding of the effect of assembly parameters on the disruptors, we altered some of the assembly parameters, for fabricating single NW resonators as well as transparent electrodes made of chained NWs and studied the effect of these changes on the frequency of the applied field in order to reduce the disruptors and generate strong director forces. Table 2. (a) and (b) summarizes the effect of changing electrode gap size and NW suspension on frequencies that generate minimum electroosmotic force (Fmin EO), minimum electrode polarization effect (Fmin EP), and maximum DEP force (Fcut-off) for nanoresonator fabrication. Table 3. (a) and (b) shows the effect of electrode gap sizes and the thickness of the polymeric film for NW chaining for transparent electrode fabrication. Our experimental results were completely aligned with the theoretical analysis presented here. Table 2. Effect of assembly parameters on frequency of disruptor and director forces and effects for nanoresonator fabrication with (a) fixed NW suspension (ethanol) with different electrode gap sizes and (b) fixed electrode gap size (10 µm) and two different NW suspensions (b)

(a) Gap

Fmin EO

Fmin EP

Fcut-off

Suspension

Fmin EO

Fmin EP

Fcut-off

(µm)

(KHz)

(KHz)

(KHz)

(σ Sm-1)

(KHz)

(KHz)

(KHz)

6

80

200

200

DI water

300

2000

2000

(5*10-3) 20

100

100

8

50

150

150

10

20

100

100

Ethanol (2.19*10-5)

Table 3. Effect of assembly parameters on frequency of disruptor and director forces and effects for NW chaining generation with (a) fixed NW suspension (2-propanol) with different electrode gap sizes and (b) fixed electrode gap size (240 µm) and different thicknesses for polymeric film. Zero thickness means NW chains formed directly on the glass substrate (rigid transparent electrode) (b)

(a) Gap

4.

Fmin EO

Fmin EP

Fcut-off

Film

Fmin EO

Fmin EP

Fcut-off

(KHz)

thickness(µm)

(KHz)

(KHz)

(KHz)

(µm)

(KHz)

(KHz)

120

4

20

20

0

1

10

10

180

2.5

15

15

10

50

100

100

240

1

10

10

25

170

1000

1000

Conclusion

Field-assisted NW positioning is a scalable and low-cost method but several variable parameters and values are involved in the process that can direct or disrupt NW positioning process. We show that improper choice of parameters and physical values can introduce forces and effects that prevent NW assembly. A guideline based on simple mathematical analysis is presented for choosing the best positioning parameters. Using this guideline, we were able to assemble, with high-yield, arrays of nanoresonators that can be used for sensitive mass detection. We have also shown how large-size rigid and flexible transparent electrodes with high light transmissivity and electrical conductivity were fabricated, using ordered multiple chains of NWs.

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Acknowledgements This work was financially supported by Canadian agencies NSERC, CFI, BCKDF and MITACS. This work was made possible by the use of the facilities of CAMTEC at the University of Victoria and of 4D LABS at Simon Fraser University. References [1]

M. Li, R.B. Bhiladvala, T.J. Morrow, J.A. Sioss, K.-K. Lew, J.M. Redwing, C.D. Keating, T.S. Mayer, Nat. Nanotechnol. 3 (2008) 88– 92.

[2]

S. Hong, H. Lee, J. Lee, J. Kwon, S. Han, Y.D. Suh, H. Cho, J. Shin, J. Yeo, S.H. Ko, Adv. Mater. 27 (2015) 4744–4751.

[3]

E.C. Garnett, M.L. Brongersma, Y. Cui, M.D. McGehee, Nanowire Solar Cells, Annu. Rev. Mater. Res. 41 (2011) 269–295.

[4]

B.P. Timko, T. Cohen-Karni, Q. Qing, B. Tian, C.M. Lieber, IEEE Trans. Nanotechnol. 9.3 (2010) 269-280.

[5]

D. Mijatovic, J.C.T. Eijkel, A. van den Berg, Lab Chip. 5 (2005) 492-500.

[6]

M. Tanase, L. A. Bauer, A. Hultgren, D. M. Silevitch, L. Sun, D. H. Reich, P. C. Searson, G. J. Meyer, (2001) Nano Lett. 1.3 (2001)

[7]

A. Pevzner, Y. Engel, R. Elnathan, T. Ducobni, M. Ben-Ishai, K. Reddy, N. Shpaisman, A. Tsukernik, M. Oksman, F. Patolsky, , Nano

[8]

E.M. Freer, O. Grachev, X. Duan, S. Martin, D.P. Stumbo, Nat. Nanotechnol. 5 (2010) 525–530.

155-158. Lett. 10 (2010) 1202–1208. [9]

M. Collet, S. Salomon, N.Y. Klein, F. Seichepine, C. Vieu, L. Nicu, G. Larrieu, Adv. Mater. 27 (2015) 1268–1273.

[10]

N.K.R. Palapati, E. Pomerantseva, A. Subramanian, Nanoscale. 7 (2015) 3109–3116.

[11]

M. Sam, J. Alsaif, R.B. Bhiladvala, SID Symp. Dig. Tech. Pap. 48 (2017) 1463–1465.

[12]

M. Sam, N. Moghimian, R.B. Bhiladvala, Mater. Lett. 163 (2016) 205–208.

[13]

M. Sam, N. Moghimian, R.B. Bhiladvala, Nanoscale. 8 (2016) 889–900.

[14]

T.B. Jones, Electromechanics of Particles, Cambridge Uninersity, Cambridge, 1995.

[15]

N.G. Green, A. Ramos, A. Gonzá Lez, H. Morgan, A. Castellanos, Physical review E 61 (2000) 4011.

[16]

N. Moghimian, M. Sam, R.B. Bhiladvala, Mater. Lett. 113 (2013) 152–155.

[17]

C. Zhang, K. Khoshmanesh, A. Mitchell, K. Kalantar-zadeh, Anal. Bioanal. Chem. 396 (2010) 401–420.

[18]

Y. T. Yang, C. Callegari, X. L. Feng, K. L. Ekinci, M. L. Roukes, Nano lett. 6 (2006) 583-586.

[19]

J.A. Sioss, R.B. Bhiladvala, W. Pan, M. Li, S. Patrick, P. Xin, S.L. Dean, C.D. Keating, T.S. Mayer, G.A. Clawson, Biol. Med. 8 (2012) 1017–1025.