Self-Assembly of Silver Nanowire Ring Structures Driven by the

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Self-assembly of silver nanowire ring structures driven by compressive force of liquid droplet Baekhoon Seong, Hyun Sung Park, Ilkyung Chae, Hyungdong Lee, Xiaofeng Wang, Hyung-Seok Jang, Jaehyuck Jung, Changgu Lee, Liwei Lin, and Doyoung Byun Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00361 • Publication Date (Web): 13 Mar 2017 Downloaded from http://pubs.acs.org on March 15, 2017

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Self-assembly of silver nanowire ring structures driven by compressive force of liquid droplet Baekhoon Seong†,‡, Hyun Sung Park§, Ilkyeong Chae†, Hyungdong Lee†, Xiaofeng Wang†, Hyung-Seok Jang§, Jaehyuck Jung∥, Changgu Lee†,∥, Liwei Lin§ and Doyoung Byun†,*

† Department of Mechanical Engineering, Sungkyunkwan university, 2066 Seobu-Ro, JanganGu, Suwon, Gyeonggi, 440-746, Republic of Korea ‡ Korea Institute of Ocean Science and Technology, Ansan 426-444, Republic of Korea § Department of Mechanical Engineering, University of California, Berkeley, 94720, United States ∥ SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, 2066 Seobu-ro, Suwon, Gyeonggi, 440-746, Republic of Korea *Corresponding author: Doyung Byun, tel:+82-31-299-4846, email: [email protected]

KEYWORDS compressive force; silver nanowire; ring structure; droplet;

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ABSTRACT

In nanowire-dispersed in liquid droplets, the interplay between the surface tension of the liquid and the elasticity of the nanowire determines the final morphology of the bent or buckled nanowire. Here, we investigate the fabrication of a silver nanowire (Ag NW) ring generated by encapsulation inside fine droplets. We used a hybrid aerodynamic and electrostatic atomization method to ensure the generation of droplets with scalable size in the necessary regime for ring formation. We analytically identify the compressive force of the droplet driven by surface tension as the key mechanism for the self-assembly of ring structures. Thus for potential largescale manufacturing, the droplet size provides a convenient parameter to control the realization of ring structures from nanowires.


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1. INTRODUCTION

Several forces in the liquid droplet drive the nanomaterials to naturally form an assembled pattern. The assembled patterns have different morphologies according to various parameters, such as temperature 1, liquid properties 2, nanomaterial size

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and shape 4, and droplet size 5.

Typically, the behavior of nanomaterials is dominated by internal flow in the droplet. During evaporation of a liquid droplet, nanomaterials can move to the rim of the droplet by convective flow and capillary flow, due to the difference in temperature between the top and contact line of the droplet. Inside the droplet, nanomaterials can self-assemble by Marangoni flow as the coffeering effect 6. This phenomenon is undesirable for uniform deposition in numerous analytical and industrial applications. Therefore, there have been various attempts to eliminate the coffee ring effect for a uniform arrangement of nanomaterials

7, 8

. In particular, small droplet size can

alleviate the problem, as the evaporation rate of the solution is faster than the rate of diffusion of nanomaterials 9. In small-sized droplet without the coffee-ring effect, the morphology of assembled nanomaterials is dominated by the compressive force of the droplet, because of the high-pressure difference across the droplet/air interface 10, 11. The small-sized droplet drives quite different phenomena for the assembly of nanomaterials than the large-sized droplet. Recently, several assembled structures have been introduced based on nanowires dispersed in small-sized droplets. The nanowire captured in the small-sized droplet can readily be deformed into various shapes because it can be of longer length than the droplet diameter. Thus, many researchers have introduced deformation of the nanowire into the desired structure along the droplet shape: coiling of colloidal gold nanowires 12 and CNTs (carbon nanotubes) of 500 nm in diameter induced by the contraction of their polymer shells 13, and copper nanocoils of 10–35 µm in diameter synthesized by a solvothermal method

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. These coiled gold nanowires and CNTs


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would go through the uncoiling process as the polymer structure was removed, while the copper nanocoil process is limited to specific materials. The coiled ring-shaped structures can be applied in tunable plasmonic resonance, due to selective penetrations of electromagnetic fields through the ring cavity. Applications of ring-shaped nanomaterials can be found in various other fields, such as superconducting structures 15, biosensors

16, 17

, solar cells 18, and optical absorbers 19. In

particular, silver nanowires (Ag NWs) have been suggested as potential ring-shaped nanomaterials, due to their outstanding electrical conductivity and high surface plasmon resonance (SPR)

20, 21, 22

. Thus, a few researchers have tried to fabricate the deformed Ag NWs

with comprehensive theories and experiments

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, including a previous report on Ag-pTh

(polythiophene) composite nanospirals structures 24. This was a very valuable study and provided evidence that Ag NWs could be deformed into perfect ring-shaped structure. Herein, we demonstrate a new, simple and scalable technology for the fabrication of ringshaped Ag NWs by a spraying method. We observed various deformed morphologies of Ag NWs to determine the regimes. We investigated the progress of ring shape formation of Ag NWs according to the droplet size with theoretically calculated optimal conditions. As such, this selfassembly technique of making ring-shaped structures from Ag NWs could be applied to other nanomaterials.

2. EXPERIMENTAL SECTION

Materials preparation. The Ag NWs were dispersed as 1 wt % of concentration with 40 nm diameter and 25 ± 5 µm length in D.I. water (Nandb Inc., Korea). Polyvinylpyrrolidone (PVP) with a molecular weight of 40,000 and polyethylene oxide (PEO) with a molecular weight of


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100,000 were purchased from Sigma-Aldrich. D.I. water-based NW solutions were mixed with 1.4 wt% PVP and 1 wt% PEO to fabricate Ag NW rings with a polymer-filled cavity. Ag NW deposition. A hybrid aerodynamic and electrostatic spraying system was installed to deposit Ag NWs (ENJET Inc., www.enjet.co.kr)25. This system consisted of a nozzle, syringe pump, gas pressure controller and high voltage unit. A silicon (Si) substrate was utilized to deposit Ag NWs, and the nozzle was set in a direction parallel to the substrate (Supplementary Fig. 1). The distance between the nozzle and substrate was 10 cm in the parallel direction, and 5 cm in the vertical direction. Ag NW solution was supplied by a syringe pump (Tricontinent Inc.) with a 1 ml/min flow rate. Compressed air was applied by a pressure controller, and high-voltage equipment (Ultravolt Inc.) was connected to a nozzle to generate a strong electric field from 0 to 10 kV. Sheath air flow was supplied at 4.4 ml/min into a gas nozzle and the flow rate was measured using a mass flow meter (SMC Corp.). The internal mixing nozzle unit consists of a liquid nozzle and a gas nozzle, in which the solution was first atomized by colliding with gas. While atomized droplets produced by an aerodynamic force fly straight through air streamlines, charged droplets sequentially atomize into finer droplets due to coulomb fission. Among them, smaller droplets are barely affected by inertia force, and thus are pulled toward the Si substrate by the electric field. Analysis of deformed NW. Deposited Ag NWs were observed on Si water by scanning electron microscopy (SEM) imaging using a Jeol JSM-7600F at an acceleration voltage of 15 kV. Based on SEM images, equivalent diameters and curvatures of NWs were geometrically measured by the least-squares fitting of circles method. The thickness of Ag NW rings was measured by atomic force microscopy (AFM) (Nanonavi Inc.).


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3. RESULTS AND DISCUSSION

Elastic materials are known to restore back toward their equilibrium position if the external force is within the limit of elastic deformation. In practice, long and thin materials tend to be readily deformable due to their high elastic modulus, including Ag NWs, due to their onedimensional shape and high aspect ratio 26. In order to study the behavior of deposited Ag NWs inside small-sized droplets, we deposited droplets containing Ag NWs solution mixtures using a hybrid aerodynamic and electrostatic atomization system (Figures S1 and S2) to have controllable droplet sizes between 0.9 and 16 µm in large-scale fabrication. Experimental results have shown that the deposited Ag NWs have various bent or buckled shapes depending on the droplet size (Figure 1a). In general, during the droplet evaporation process, the surface tension force on the boundary of the droplet may bend the Ag NWs, and sometimes force the NWs to shape along the rims of the droplet. We systemically investigated the Ag NW deformation morphologies with respect to the droplet size. For large droplets, the conventional coffee ring effect dominates, such that we found layers of NWs piled up at the edges (Figure 1b). Smaller droplets can lead to a higher evaporation rate than the diffusive particle movement rate, avoiding the coffee ring effect, as described in a previous report 9. Experimentally, we found NWs to be deposited and deformed or coiled inside the original circular droplet configurations (Figures 1c and d) for small droplets. Interestingly, under our experimental conditions, near-perfect ring shapes from deformed NWs can form as the droplet diameter falls below a specific diameter (Figure 1e). Many parameters affect the final shapes of the NWs, such as the diameter, length, and properties of the NWs 27, and the size, composition and properties of the droplets. Intuitively, if the size of the droplet is much larger than the dimensions of the NWs, the conventional coffee ring effect of accumulating NWs at the edges of the droplet is expected

28

. As the droplet


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diameter reduces to be smaller than the length of the NWs, the surface tension force may deform the NWs to various shapes, including simple ring structures. Figures 2a-c are scanning electron microscope (SEM) images of various depositions results. In Figure 2a, 8 ring structures made of Ag NWs can be identified with the size of 2.17±0.25 µm in diameter within the area of 45 × 60 µm2, and Figure 2b is a close-up SEM photo of the same deposition result taken from a different location. In both images, we observe straight and severely distorted NWs. Some of these long NWs may have been: (1) left out of the droplets in the droplet formation/separation process, and so maintained their original straight shape; (2) deformed, but overcame the droplet evaporation process, and restored to their original, straight shape; or (3) severely deformed for permanent plastic deformation as the droplet landed on the deposition surface. In addition, the relatively small length of Ag NWs could have folded (buckled), because of their low elastic modulus 29, 30. Figure 2c shows the deposition results of a wider distribution of droplet sizes with either simple ring structures of 1.5 to 2.2 µm in diameter, or randomly coiled structures inside the circular boundary of larger droplet sizes of 4 to 6.5 µm in diameter. To determine the distinct regimes Dr that result in the formation of a ring shape structure, we experimentally investigated the non-dimensional curvatures of the deformed Ag NWs according to the droplet size (Figure 2d). We defined the equivalent diameter of the droplet as Dds, radius as rds, and the deformed curvature as R (Figure S3). The rds/R values of the ring shape Ag NWs were close to 1, while the rds/R values of the randomly kinked Ag NWs were greater than 1. Under the experimental conditions with Ag NWs of 40 nm in diameter and 20±5 µm in length (Figure S4), we found that ring shape structures can be constructed if the droplet diameter is between 0.9 and 4.25 µm. Results also show that perfectly coiled Ag NW rings (rds/R ≈ 1) and randomly kinked Ag NWs (rds/R > 1) could coexist in equivalent droplet diameter, Dds,


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from 2.37 to 4.25 µm. In the range of randomly kinked NWs, the value of rds/R became much larger than 1 and gradually increased with increasing droplet sizes. In small sized droplet, the Ag NW can’t be affected by internal flow, and has restoring force when its length is larger than the droplet size. Thus to theoretically analyze the deformation of Ag NW, we can consider the bending force of Ag NW and compressive force induced by the surface tension of droplet (Figure 3a). Analytically, the maximum deflection ( ) of NW inside the droplet can be estimated from the simply supported beam deflection model and is given by:

δ max ( x) = − qnet =

5qnet Lp 4 384 EI

=−

5qnet Dds 4 384 EI

(1)

Fr F = r L p Dds

(2)

where E and I are Young’s modulus of Ag NW

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and the area of momentum inertia,

respectively. Lp is the distance between the two points of support, which is equal to the diameter of the wetted droplet Dds (Lp=Dds), and qnet is the uniformly distributed load per unit length. When the NW deforms as a perfect ring shape along the circumference of a droplet, the maximum deflection of Ag NW reaches the radius of the wetted droplet. Thus, we can replace

δ max with the radius of a wetted droplet (D /2) to compute the bending force of NW. By ds substituting this wetted droplet radius (Dds/2) into the expression for maximum deflection ( ), we can rearrange the bending force FR for ring formation as:

FR =

384 EI 10 Dds 2

(3)

We assign that the bending force is the restoring force of the NW, because the hydrostatic force inside a liquid droplet doesn’t need to be considered. The surface tension of the liquid


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induces a pressure difference across the droplet interface (Figure 3b). This pressure induced from capillary effect compresses the nanomaterials

31, 32

. When the Ag NW deforms inside a droplet

and is bent by the droplet interface, the surface tension force is exerted on the surface of the NW with the overall net force: FC = ANW ∆pdrop ANW = 2 ∫

θ Area

360 θ =0

π

(4)

rNW LNW sin θ dθ

(5)

where Anw is the area of the exerting force on the NW. Anw can be determined when the droplet endures the restoring NW by the surface tension. Anw is determined by θArea (87.4°), which is calculated from the contact (θ1, 70°), advancing (θ2, 89°), and receding (θ3, 51°) angle of the liquid droplet on the Ag plate, and the contact angle (θ3, 40°) of the liquid droplet on Si substrate (Figure 3c). ∆ is the pressure difference across the droplet interface by the Young-Laplace equation 33: 2γ Rdrop

∆Pdrop =

where,

(6)

Rdrop

is the radius of droplet curvature, and we can estimate this radius from the

geometric relation between the diameter of the wetted droplet, and the contact angle of the droplet (Figure S5). The pressure difference exponentially changes according to the droplet size (Figure S6). By combining Equations (4), (5) and (6), we obtain the net force acting on the NW in a radial direction: θ Area

FC = 4∫ 360 θ =0

FC = 0.684

π

γ rnw lnw Rdrop

sinθ dθ (7)

4 Lnw rnwγ Rdrop

(8)


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Confinement of NWs inside a droplet is a prerequisite for ring formation. If the length of the Ag NW does not reach the critical length for beam buckling, Ag NWs will not bend, and in some cases, will puncture the liquid interface 34. Thus to confine NW inside the droplet, Fnet should be larger than the critical load for buckling of the NW. The Euler buckling load for a simply supported NW can be solved as 34:

FB =

π 2 EI Lnw2

(9)

Equation (9) shows that the critical load (FB) for buckling is inversely proportional to the length of the beam. Therefore, we can estimate the condition in which the Ag NWs will freely bend or buckle inside a droplet by comparing FC with FB (Figure S7). We theoretically determine the distinct regime for ring formation by comparing the force balance between the compressive force of the droplet and the restoring force of the NW. In a perfect ring formation, the NW should coil along the circumference of the droplet, because its restoring force is not sufficient to puncture the droplet, and it tends to be directed back toward the equilibrium position. Otherwise, if the surface tension force exerted on the NW is strong enough, the NW randomly kinks inside the droplet, due to the instability of its position. If a stronger compressive force (FC > FR) is exerted, the NW can more strongly bend or buckle from the ring shape. Thus, to form a perfect ring-shaped Ag NW, the exerted force FC should be smaller than the restoring force FR. Comparison between the compressive force FC and the required force FR for ring formation with respect to the wetted droplet size can determine the regimes required to generate particular shapes of deformed Ag NWs (Figure 3d). If the FR is stronger than FC, the Ag NW will bend along the circumference of the droplet due to the restoring force. The distinct section of ring shape, in which FC is lower than FR, is indicated in wetted droplets

Self-Assembly of Silver Nanowire Ring Structures Driven by the

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