Shell Nanowire Surface Fastener Used for

Oct 12, 2013 - In this paper, a room-temperature electrical surface fastener consisting ... One typical example is the hook and loop structure in burd...
4 downloads 0 Views 5MB Size
Download PDF
Article pubs.acs.org/Langmuir

Copper/Parylene Core/Shell Nanowire Surface Fastener Used for Room-Temperature Electrical Bonding Peng Wang, Yang Ju,* Yanbin Cui, and Atsushi Hosoi Department of Mechanical Science and Engineering, Nagoya University, Nagoya 464-8603, Japan S Supporting Information *

ABSTRACT: The traditional bonding technology in electronic assembly relies on high-temperature processes, such as reflow soldering or curing of adhesives, which result in undesired thermal excursions and residual stress at the bonding interface. Therefore, there is an urgent need to attach electronic components on the circuit board with good mechanical and electrical properties at room temperature. In this paper, a room-temperature electrical surface fastener consisting of copper/parylene core/shell nanowire (NW) arrays were prepared, and van der Waals (VDW) forces were utilized to interconnect the core/shell NWs. Interestingly, the Parylene C film becomes conductive due to dielectric breakdown when the thickness of it is miniaturized to nanoscale. Our electrical surface fastener exhibits high macroscopic adhesion strength (∼25 N/cm2) and low electrical resistance (∼4.22 × 10−2 Ω·cm2). Meanwhile, a new theoretical model based on VDW forces between the NWs is proposed to explain the adhesion mechanism of the core/shell structure.

■

carried out. Carbon nanofibers (CNFs),22 single-walled CNTs,23 and multiwalled CNTs,24 have shown a relatively high adhesion strength23 or a relatively low electrical resistance.24 Also, the Ge/parylene core/shell nanowire array can also be used as an electrical connector after the deposition of a Ag film with a relatively high shear adhesion strength.25 In addition, Ni26 and Cu27 nanowire arrays have been found to have very low electrical resistance. Moreover, we have reported nanowire surface fasteners (NSFs) based on gold and copper nanowire arrays,28,29 with relatively low electrical resistance and simultaneous shear and normal adhesion strengths. Compared with traditional high-temperature electrical bonding technologies (EBT), room-temperature EBT has many advantages. However, to achieve high adhesive strength and low electrical resistance at the same time is still a challenge. In this work, we fabricated copper/parylene core/shell NSFs. Parylene C [poly(cholro-p-xylylene), −H2CC6H3ClCH2−)n] is widely used in microelectronics as the nonconductive cover film. However, it was shown in this paper that the dielectric breakdown makes it become conductive as the thickness shrank to nanoscale. Here, a new cell for electrodeposition was adopted so that copper NWs from polycarbonate template could grow on substrate directly. After coating a thin parylene film on the copper NWs, this NSF showed an adhesive strength of ∼25 N/cm2 and an electrical resistance of ∼4.2 × 10−2 Ω· cm2. In addition, a new model was developed to analyze the van

INTRODUCTION The continuous trend toward miniaturization and functional density enhancement makes urgent the demand to improve the bonding technology in electronic assembly. Right now electronic assembly relies on high-temperature processes such as reflow soldering or curing of adhesives. The traditional Pb− Sn solder with a composition of 62 wt % Sn and 38 wt % Pb has a melting point of 183 °C.1,2 Many kinds of Pb-free solder have been adopted, but their melting points are always 5−20 °C higher than those of Pb−Sn solder.3 The high-temperature tends to result in undesired thermal excursions and residual stress at the bonding interface,4 which not only lead to reliability issues but also restrict the application of temperaturesensitive components. Therefore, attaching components on the circuit board with good mechanical and electrical properties at room temperature is always a goal of researchers. On the other hand, nature has created a unique structured device in the form of a mechanical interlocker. One typical example is the hook and loop structure in burdock seeds, which led to the invention of Velcro.5 Moreover, interlock is also achieved in beetle’s wing-fixation device by bring densely populated microhairs on the cuticular surface in interconnection.6 Motivated by these observations, polymer nanofibers,7−12 core/shell-type nanowires,13 Janus nanopillars,14,15 hierarchical structure fibers,16,17 smart tip structures,18,19 and multiwalled carbon nanotube (CNT)20,21 have been introduced as a permanent or reversible adhesive. However, electrical connection was not realized or reported in the aforementioned works. Recently, some challenges to realize electrical connection using nanowire or nanotube arrays have been © 2013 American Chemical Society

Received: July 1, 2013 Revised: October 12, 2013 Published: October 12, 2013 13909

dx.doi.org/10.1021/la402475f | Langmuir 2013, 29, 13909−13916

Langmuir

Article

Figure 1. (a) Schematic for the electrical resistance measurement of thin parylene film. (b) The electrical resistance of parylene film as a function of thickness. glass plate ensures the contact of the substrate with PC membrane. Third, the compliance of porous cellulose membrane offers a buffer and then ensures tight contact of the substrate with PC membrane. It is known that for flat anodes the metal is deposited preferentially at the outer border areas of the cathode.31 This effect was avoided by using a conical copper anode, leading to a noticeably more homogeneous copper nanowire distribution over the whole cathode surface.32 Before and after the assembly of the cell, two additional immersions were introduced to ensure an even copper ion density throughout the PC membrane.29 Copper nanowire arrays were then synthesized by electrodepostion under a constant current of approximately 3 mA. The electrodeposition electrolyte used was a 0.4 M CuSO4·5H2O solution, adjusted to pH 2 with sulfuric acid. The electrodeposition was performed at room temperature and without stir. The interelectrode distance was kept at around 40 mm. After etching in methylene chloride to remove the PC membrane, the freestanding copper nanowire arrays on the substrate were obtained. Parylene Coating. A thin film of Parylene C was deposited on copper nanowire arrays by using a DACS-LAB deposition system. The typical deposition conditions were 160 °C for the evaporation of the parylene dimer precursor, 650 °C for the pyrogenic decomposition of the dimer into monomers, and 60 mTorr for the vacuum chamber. Through controlling the amount of the loaded precursor, the corresponding thickness of parylene shell was obtained. Testing of Adhesive Strength and Electrical Resistance. To investigate the adhesive strength, an external preload was applied by pressing two NSF samples against each other. After the preload was released, the weight of balance was used to measure the pull-off forces with the parallel (i.e., the shear adhesive strength) and normal (i.e., the normal adhesive strength) directions to the glass substrate. The experimental sketch is shown in Figure S2 (Supporting Information). To measure the electrical resistance, the four-point probe method24 shown in Figure S3 (Supporting Information) was used. The current in the range from 0 to 2 mA was applied by a constant current source and the corresponding voltage was extracted from a voltmeter.

der Waals forces between the core/shell NWs, which can explain the adhesive mechanism of NSFs.

■

EXPERIMENTAL SECTION

The Electrical Testing of Thin Parylene Film. Samples were fabricated on 28 × 10 mm glass substrate over which a continuous layer of Cr/Au (50/200 nm thick, respectively) was sputtered to serve as the ground. With the help of mask, a specific thickness of the Parylene C film was then deposited over the central area of the substrate. To create the upper contact electrodes, a thin gold film (100 nm thick) was deposited on the parylene film in a specific pattern. As shown in Figure 1a, the electrical resistance measurement was performed and the current from 0 to 2 mA was applied by a constant current source to the four-point probe measuring circuits. The corresponding voltage was extracted from a voltmeter. Pattern Design and Fabrication. A specific pattern for the fastener areas and printed wires was designed to facilitate the mechanical and electrical testing, as shown in Figure S1 (Supporting Information). The diameter of each of the four fastener areas was 2 mm. With the help of mask, the pattern was fabricated by sputtering a 50 nm Cr adhesion layer and 100 nm Au seed layer. Electrodeposition of Copper Nanowires. Polycarbonate (PC) membranes (ISOPORE, Millipore Inc.) with 150 nm diameter pores measured on average were used as the templates. The peculiar cell shown in Figure 2 was used to fabricate free-standing copper nanowire on the substrate directly. The introduced porous glass and porous cellulose membrane have three important functions. First, the capillary forces provided by the porous glass plate and porous cellulose membrane help maintain a continuous electrolyte flow from the bulk of the electrolyte to PC membrane.30 Second, the stiffness of porous

■

RESULTS AND DISCUSSION Figure 1b shows the measured electrical resistance of the Parylene C films with different thicknesses (see Supporting Information S2 for details). Because the parylene film is nonconductive on a microscale, it is easy to understand the tendency in Figure 1b that larger parylene thicknesses result in larger electrical resistance. Moreover, since a constant current source was used to measure the resistance of the parylene films, the initial voltage applied to the film tended to reach the breakdown voltage so as to flow the required current. Thus, the parylene film was dielectrically broken down and became conductive. The reason why Parylene C film in nanoscale thickness becomes conductive is mainly due to the dielectric breakdown

Figure 2. Schematic of the cell for copper nanowire fabrication: (a) conical copper anode, (b) porous glass plate, (c) cellulose membrane, (d) polycarbonate template, (e) glass substrate with gold film, (f) isolation holder, (g) screw and nut, and (h) copper wire. 13910

dx.doi.org/10.1021/la402475f | Langmuir 2013, 29, 13909−13916

Langmuir

Article

Figure 3. (a) Schematics of the fabrication process. SEM images of copper NWs (b) without parylene coating, (c) with 100 nm parylene coating, and (d) with 200 nm parylene coating. The red arrows in parts c and d indicate the parylene shell. The scale bar is 1 μm.

phenomenon. For parylene film with a thickness larger than 1 μm, the breakdown voltage had been estimated by the equation Vbd = 1.89h0.45, based on the experimental data,33 where Vbd is the breakdown voltage in volts and h is the thickness in micrometers. According to this equation, the breakdown voltage of 300 nm thick parylene film could be 1.10 V. However, the crystal size at low thickness is smaller than that at high thickness,34 and the voids and discontinuities that may exist in the material will have an effect as the film thickness decreases. Therefore, the thickness in nanoscale will further decrease the breakdown voltage. In this study, the breakdown voltage of the Parylene C film with 300 nm thickness was measured to be 0.55 V (see Supporting Information S9). The fabrication procedure of the copper/parylene core/shell NSFs is outlined in Figure 3a. At first, copper NWs were grown on glass/Cr/Au substrates by the template-assisted electrodeposition method. After etching the PC template, a thin layer of Parylene C was evenly deposited on the copper NWs to enhance the adhesive ability of NSFs. The scanning electron microscopy (SEM) image of the copper NW arrays with an average diameter of 150 nm (Figure 3b) indicates that most of the NWs were grown vertically on the substrate but oriented in a wide range of directions. Parts c and d of Figure 3 show the SEM image of copper NWs with a 100 and 200 nm parylene coating, respectively. Clearly, the grown copper NWs sustain their high aspect ratio without aggregation, partly due to the high Young’s modulus of the copper (∼110 GPa). To characterize the properties of core/shell NSFs, we first measured the adhesive strength and relative electrical resistance as a function of NW length (5, 10, and 20 μm). All the samples in this test have a parylene shell (100 nm thickness) and experienced a preload of 78.02 N/cm2. As can be seen in Figure 4a, both shear and normal adhesion can be realized at the same time, which is different from other kinds of core/shell-type connectors.13,25̀ Moreover, the adhesive strength is strongly affected by the length of the NWs. The maximum shear and normal strengths were obtained when the length was 10 μm. When L < 10 μm, the NWs sustained their high aspect ratio and the neighboring NWs did not contact with each other (Figure S5a, Supporting Information); therefore, the contact area is directly proportional to the NW length. However, when the NW length is as large as 20 μm, the NWs tend to collapse

and the neighboring NWs contact with each other (Figure S5c, Supporting Information), which leads to a reduction of the contact area of the NSFs. The electrical resistance is also strongly affected by the length of NWs. Specifically, longer NW length results in smaller electrical resistance (Figure 4b). As can be seen in Figure S5a−c (Supporting Information), the interconnection of neighboring NWs increases as the length of NWs increases, and they were interconnected before the parylene coating. The interconnected neighboring NWs connect in parallel in the electrical connection, which led to the reduction of resistance. Besides the nanowire length, the adhesive strength and electrical resistance are also affected by the parylene thickness. The samples with the same nanowire length (10 μm) and the same preload (78.02 N/cm2) are used in the test. As can be seen in Figure 4c, the adhesive properties strongly depend on the thickness of the parylene shell. Specifically, weak adhesive strengths (∼0.99 N/cm2 in shear and ∼0.57 N/cm2 in normal directions) are obtained from the pristine copper NWs. The adhesive strength is dramatically enhanced by the application of the parylene shell. When the thickness of the parylene shell is 150 nm, the maximum adhesive strengths (∼24.97 N/cm2 in shear and ∼10.82 N/cm2 in normal directions) are obtained. This significant enhancement in adhesion is attributed to the higher surface compliance of the parylene shell, enabling conformal contact with increased contact area between the interpenetrating NWs.13 When the thickness of parylene shell further increases, the adhesive strengths decrease. This trend is attributed to the higher filling factor for thicker parylene shells (Figure 3b−d).13 When the thickness of the parylene shell increases to 250 nm (Figure S6, Supporting Information), almost no spare space exists between the neighboring NWs. Hence, the interconnected mode changes from “wire−wire” to “tip−tip” when the thickness of paryelene shell increases, which results in the reduction of adhesive strengths. The electrical properties of NSFs are also affected by the parylene shell thickness. It can be seen from Figure 4d that larger parylene shell thickness results in larger electrical resistance of the NSFs. This trend is attributed to the poor electrical conductivity of parylene. To examine the effect of preload on the adhesive and electrical properties of core/shell NSF, two NSF samples were 13911

dx.doi.org/10.1021/la402475f | Langmuir 2013, 29, 13909−13916

Langmuir

Article

Figure 4. (a) Adhesive strength and (b) electrical resistance of NSFs as a function of nanowire length. The preload is 78.02 N/cm2 and the thickness of the parylene shell is 100 nm. (c) Adhesive strength and (d) electrical resistance of NSFs as a function of parylene thickness. The preload is 78.02 N/cm2 and the length of the nanowire array is 10 μm. (e) Adhesive strength and (f) electrical resistance of NSFs as a function of preload. The thickness of parylene shell is 150 nm and the length of nanowire array is 10 μm.

the red light from the light-emitting diode shows that the core/ shell NSFs are conductive. Finally, the results obtained in this study are summarized in Table 1, which compares our data with those reported by others. In addition, in order to further understand the interlocking behavior presented here, a simple theoretical model is adopted to quantitatively describe the measured adhesive forces. Once the nanowires are brought into interconnection at a uniformed preload, the VDW forces between the interconnected nanowires are assumed to contribute to the bonding. As shown in Figure 6a, the interconnected nanowires were crossed at arbitrary angles. Considering two extreme interconnection scenarios between two cylinders, arbitrary interconnection (Figure 6c) can be considered in either parallel (Figure 6b) or perpendicular (Figure 6d) configuration with each other. Until now, the expressions for the VDW forces between two core/ shell cylinders were not available. On the basis of the

brought into interconnection at a preload of 39.01, 78.02, and 156.04 N/cm2. A monotonic increase in the normal adhesive strength and decrease in electrical resistance are observed with the increase of preload force (Figure 4e,f). This phenomenon is just as expected because the higher preload force leads to a larger contact area between the NWs. However, no increase in the shear adhesion was observed when the preload increases from 78.02 to 156.04 N/cm2. This phenomenon is attributed to the poor adhesion of electrodeposited copper NWs on the Au seed layer. As can be seen in Figure S7a (Supporting Information), NW arrays detached from the Au seed layer after the adhesion test. Figure 5a shows the schematic of copper/parylene core/shell NSFs. An example of the strong bonding achieved is shown in Figure 5b in which the copper/parylene core/shell NSFs with a surface area of ∼3.14 × 4 mm2 enables 300 g of weight to be hung without failure in shear direction. As shown in Figure 5c, 13912

dx.doi.org/10.1021/la402475f | Langmuir 2013, 29, 13909−13916

Langmuir

Article

where E(x, y) is the interaction energy between a geometrical object x and a geometrical object y. Moreover, the interaction energy per unit length between one cylinder core and one cylinder shell can be formulated as E(core1, shell 2) = E(core1, cylinder2) − E(core1, core2) (2)

where the indexes 1 and 2 can be reversed. The VDW forces, corresponding to the interaction energy, can be defined as36 F = −dE /dD

(3)

Here, D is the shortest distance between the surfaces of cylinders. Then, through substituting eqs 1 and 2 into eq 3, we can further derive that F(shell1, shell 2) = F(cylinder1, cylinder2) + F(core1, core2) − F(core1, cylinder2) − F(core2 , cylinder1)

(4)

and F(core1, shell 2) = F(core1, cylinder2) − F(core1, core2) (5) Figure 5. (a) Schematic of this room-temperature electrical bonding technique. (b) Photo showing a weight of 300 g hanging on the interconnected copper NSFs. (c) Light-emitting diode suspended by the NSFs to show electrical conductivity.

where the indexes 1 and 2 can be reversed. When two solid cylinders have radii of R1 + h1 and R2 + h2, respectively, and are at a distance D apart, the VDW force F in parallel-contacting can be formulated as36

assumption of additivity, the VDW expressions between two spherical shells have been presented.35 Here, the VDW expression for core/shell cylinder mode was derived through the same method. In this study, the interconnected core/shell NWs have the same core radius and the same shell thickness. However, in order to obtain a more general result, the geometry of the interconnected core/shell cylinders was assumed to be different. As can be seen in Figure 6e, one core/shell cylinder has a core radius of R1 and shell thickness of h1, and another core/shell cylinder has a core radius of R2 and shell thickness of h2. The total interaction energy per unit length between the two cylinder shells can be formulated as

F(R1 + h1 , R 2 + h2 , D) =

(6)

Moreover, the VDW force F in perpendicular-contacting can be formulated as36 F(R1 + h1 , R 2 + h2 , D) =

A ((R1 + h1)(R 2 + h2))0.5 2 6D (7)

where A is the Hamaker constant (corresponding to the material) and l is the overlapped length. On the basis of eqs 4, 5, 6, and 7, the expression of VDW forces in some limited cases were obtained and summarized in Table 2. Consequently, the VDW forces between two core/shell cylinders can be formulated as

E(shell1, shell 2) = E(cylinder1, cylinder2) + E(core1, core2) − E(core1, cylinder2) − E(core2 , cylinder1)

0.5 Al ⎛ (R1 + h1)(R 2 + h2) ⎞ ⎟ ⎜ 8 2 D2.5 ⎝ R1 + R 2 + h1 + h2 ⎠

(1)

Table 1. Comparison of Nanowire and Nanotube Arrays Which Could Be Used for Electrical and Mechanical Connection adhesive strength (N/cm2) material CNTs and CNFs CNTs CNTs Ge/parylene/Ag NWs Ni NWs Cu NWs Au NWs Cu NWs Cu/parylene NWs

shear 15

normal 4.38 × 10 29

−2

30

5.5 8.17 24.97

5 4.10 10.82 13913

electrical resistance (Ω·cm2)

ref

1.6 ∼12.8 ∼1.24 × 10−2 ∼6.25 6 × 10−5 ∼2.75 × 10−5 6.28 × 10−2 0.69 × 10−2 4.22 × 10−2

22 23 24 25 26 27 28 29 this work

dx.doi.org/10.1021/la402475f | Langmuir 2013, 29, 13909−13916

Langmuir

Article

Figure 6. (a) SEM images of interconnected NWs. The parylene thickness is 150 nm, the preload is 78.02 N/cm2, and the length of nanowire array is 10 μm. (b) Parallel-contacting cylinders. (c) Interconnected cylinders with an arbitrary angle. (d) Perpendicular-contacting cylinders. (e) The cross-section of two core/shell cylinders.

Table 2. The VDW Forces between Some Limited Geometries

overlapped length is l ≈ 650 nm. Considering D = D0 = 0.4 nm,36 Acu = 28.4 × 10−20J,37 Apa = 10.678 × 10−20J, Apa‑cu = 17.414 × 10−20 J (see Supporting Information S6), the VDW forces (FVDW) between the parallel- and perpendicularcontacting cylinders can be obtained from the expressions for core/shell cylinders derived in this paper. The normalization by area is taken by Fshear = ρFVDW (ρ is the effective nanowires interconnect density per area, ∼3 × 108 nanowires cm−2).20 After the normalization by area, the shear strengths Fshear are

FVDW = F(shell1, shell 2) + F(core1, shell 2) + F(core2 , shell1) + F(core1, core2)

(8)

which can be solved by substituting the equations in Table 2 and eq 6 or 7 into eq 8. Although the writing equation for core/ shell cylinder is redundant and lengthy, the calculation is easy with the help of a computer. As shown in Figure 6a, the radius of copper nanowire is R ≈ 75 nm, the thickness of parylene is h ≈ 150 nm, and the 13914

dx.doi.org/10.1021/la402475f | Langmuir 2013, 29, 13909−13916

Langmuir

Article

192.906 and 7.508 N/cm2 for parallel- and perpendicularcontacting mode, respectively. For the normal adhesion, we assumed that Amonton’s first law (Ffriction = μFload) is effective.38 Considering μ = 0.369,39 Fnormal is 71.182 N/cm2 for parallelcontacting cylinders and 2.770 N/cm2 for perpendicularcontacting cylinders, respectively. The measured adhesive strengths are much smaller than the calculated value for parallel-contacting cylinders but larger than the calculated value for perpendicular-contacting cylinders. This trend can be explained by the following: (1) The actual contact is a mixture of parallel- and perpendicular-contacting cylinder modes. The calculation of parallel-contacting cylinders provided an upper limit, while the adhesion for perpendicular-contacting cylinders offered the lower limit. (2) The backing substrate is rather stiff, which leads to the stress concentration and, thus, drastically lower the adhesive strength.40 (3) The calculation values were based on an ideal model. In fact, the contact geometry (i.e., nanowire length, radius) and the contact orientation between two nanowires are nonideal. Besides van der Waals forces, there are several other factors that may affect the adhesion strength of the NSFs, such as interfacial shear strength (ISS) of the Parylene C shell, the tensile strength of the Parylene C material (TS), and the adhesion strength of copper nanowire tearing at the base from the substrate (AS). The maximum adhesion strength of the NSFs is determined by the limit forces of them; i.e., force = min(VDW, ISS, TS, AS). For the copper/parylene core/shell nanowires with 10 μm length, the maximum adhesion strength of the NSFs is determined by AS (Figure S8, Supporting Information). Therefore, AS of a single nanowire was estimated to be 2.1 μN, and the maximum adhesive strength of the NSFs was evaluated to be 630 N/cm2 (see Supporting Information S7). On the other hand, it should be noted that the fabricated NSFs also may have a potential application for transparent conducting electrode,41−43 by transferring the copper nanowires onto a transparent adhesive film (Figure S9, Supporting Information). The transmittance is possible to control by adjusting the density of the copper nanowires.

parylene shell, (S5) nanowire arrays after the adhesion testing, (S6) calculation of the Hamaker constant, (S7) determination of maximum adhesion strength, (S8) potential application for transparent conducting electrode, and (S9) breakdown voltage of Parylene C film. This material is available free of charge via the Internet at http://pubs.acs.org.

■

Corresponding Author

*Tel: (81) 052-789-4672. Fax: (81) 052-789-3109. E-mail: ju@ mech.nagoya-u.ac.jp. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Japan Society for the Promotion of Science under Grants-in-Aid for Scientific Research (A) 23246024.

■

REFERENCES

(1) Massalski, T. B. In Binary Alloy Phase Diagrams, 1st ed.; ASM Int.: Materials Park, OH, 1987; p 1848. (2) Kang, S. K.; Sarkhel, A. K. Lead (Pb)-free solders for electronic packaging. J. Electron. Mater. 1994, 23, 701−707. (3) Harrison, M. R.; Vincent, J. H.; Steen, H. A. H. Lead-free reflow soldering for electronics assembly. Solder Surf. Mount Technol. 2001, 13, 21−38. (4) Takigawa, R.; Higurashi, E.; Suga, T.; Sawada, R. Roomtemperature bonding of vertical-cavity surface-emitting laser chips on Si substrates using Au microbumps in ambient air. Appl. Phys. Express 2008, 1, 112201. (5) de Mestral, G.Velvet type fabric and method of producing same. U.S. Patent 2,717,437, 1955. (6) Gorb, S. N. Frictional surfaces of the elytra-to-body arresting mechanism in tenebrionid beetles (Coleoptera: Tenebrionidae): Design of co-opted fields of microtrichia and cuticle ultrastructure. Int. J. Insect Morphol. 1998, 27, 205−225. (7) Pang, C.; Kim, T. I.; Bae, W. G.; Kang, D.; Kim, S. M.; Suh, K. Y. Bioinspired reversible interlocker using regularly arrayed high aspectratio polymer fibers. Adv. Mater. 2012, 24, 475−479. (8) Pang, C.; Kang, D.; Kim, T. I.; Suh, K. Y. Analysis of preloaddependent reversible mechanical interlocking using beetle-inspired wing locking device. Langmuir 2012, 28, 2181−2186. (9) Lee, J.; Fearing, R. S. Wet self-cleaning of superhydrophobic microfiber adhesives formed from high density polyethylene. Langmuir 2012, 28, 15372−15377. (10) Gillies, A. G.; Fearing, R. S. Shear adhesion strength of thermoplastic gecko-inspired synthetic adhesive exceeds material limits. Langmuir 2011, 27, 11278−11281. (11) Chen, C.; Chiang, C.; Lai, C.; Xie, T.; Yang, S. Buckling-based strong dry adhesives via interlocking. Adv. Funct. Mater. 2013, 23, 3813−3823. (12) Lee, D. H.; Kim, Y.; Fearing, R. S.; Maboudian, R. Effect of fiber geometry on macroscale friction of ordered low-density polyethylene nanofiber arrays. Langmuir 2011, 27, 11008−11016. (13) Ko, H.; Lee, J.; Schubert, B. E.; Chueh, Y.; Leu, P. W.; Fearing, R. S; Javey, A. Hybrid core−shell nanowire forests as self-selective chemical connector. Nano Lett. 2009, 9, 2054−2058. (14) Choi, M. K.; Yoon, H.; Lee, K.; Shin, K. Simple fabrication of asymmetric high-aspect-ratio polymer nanopillars by reusable AAO templates. Langmuir 2011, 27, 2132−2137. (15) Yoon, H.; Woo, H.; Choi, M. K.; Suh, K. Y.; Char, K. Face selection in one-step bending of janus nanopillars. Langmuir 2010, 26, 9198−9201. (16) Jin, K.; Tian, Y.; Erickson, J. S.; Puthoff, J.; Autumn, K.; Pesika, N. S. Design and fabrication of gecko-inspired adhesives. Langmuir 2012, 28, 5737−5742.

■

CONCLUSIONS In summary, we designed a unique electrical surface fastener with strong adhesion based on the van der Waals forces between the two sets of identical core/shell nanowire arrays. The thin Parylene C film was found to be conductive as the thickness shrunk to nanoscale. Hence, both strong bonding and small electrical resistance were achieved at room temperature for this copper/parylene core/shell nanowire surface fastener. It is unique that this electrical surface fastener exhibits high macroscopic adhesion strength (∼25 N/cm2) and low electrical resistance (∼4.22 × 10−2 Ω·cm2). Furthermore, a simple theory based on van der Waals forces was used to explain the adhesive mechanism, and the measured adhesive strength is much smaller than the calculated value for parallel-contacting cylinders but larger than the calculated value for perpendicular-contacting cylinders.

■

AUTHOR INFORMATION

ASSOCIATED CONTENT

* Supporting Information S

(S1) Schematics of NSFs for mechanical and electrical testing, (S2) resistance measurement of Parylene C film, (S3) the effect of nanowire length on the adhesive strength and electrical resistance, (S4) core/shell nanowire array with 250 nm 13915

dx.doi.org/10.1021/la402475f | Langmuir 2013, 29, 13909−13916

Langmuir

Article

(17) Bhushan, B. Bioinspired structured surfaces. Langmuir 2012, 28, 1698−1714. (18) Xue, L.; Kovalev, A.; Thöle, F.; Rengarajan, G. T.; Steinhart, M.; Gorb, S. N. Tailoring normal adhesion of arrays of thermoplastic, spring-like polymer nanorods by shaping nanorod tips. Langmuir 2012, 28, 10781−10788. (19) Campo, A. D.; Greiner, C.; Arzt, E. Contact shape controls adhesion of bioinspired fibrillar surfaces. Langmuir 2007, 23, 10235− 10243. (20) Qu, L.; Dai, L.; Stone, M.; Xia, Z.; Wang, Z. L. Carbon nanotube arrays with strong shear binding-on and easy normal lifting-off. Science 2008, 322, 238−242. (21) Zhao, Y.; Tong, T.; Delzeit, L.; Kashani, A.; Meyyappan, M.; Majumdar, A. Interfacial energy and strength of multiwalled-carbonnanotube-based dry adhesive. J. Vac. Sci. Technol. B. 2006, 24, 331− 335. (22) Tong, T.; Zhao, Y.; Delzeit, L.; Kashani, A.; Majumdar, A. Multiwalled carbon nanotube/nanofiber arrays as conductive and dry adhesive interface materials. ASME 2004 3rd Integrated Nanosystems Conference; Pasadena, CA, 2004; pp 1−6. (23) Qu, L.; Dai, L. Gecko-foot-mimetic aligned single-walled carbon nanotube dry adhesives with unique electrical and thermal properties. Adv. Mater. 2007, 19, 3844−3849. (24) Park, M.; Cola, B. A.; Siegmund, T.; Xu, J.; Maschmann, M. R.; Fisher, T. S.; Kim, H. Effect of a carbon nanotube layer on electrical contact resistance between copper substrates. Nanotechnology 2006, 17, 2294−2303. (25) Kapadia, R.; Ko, H.; Chuen, Y.; Ho, J. C.; Takahashi, T.; Zhang, Z.; Javey, A. Hybrid core−multishell nanowire forests for electrical connector applications. Appl. Phys. Lett. 2009, 94, 263110. (26) Baek, S. S.; Fearing, R. S. Reducing contact resistance using compliant nickel nanowire arrays. IEEE Trans. Compon. Packag. Technol. 2008, 31, 859−868. (27) Xu, J.; Razeeb, K. M.; Sitaraman, S. K.; Mathewson, A. The fabrication of ultra long metal nanowire bumps and their application as interconnects. IEEE NANO 2012; Birmingham, UK, 2012; pp 1−6. (28) Ju, Y.; Amano, M.; Chen, M. Mechanical and electrical cold bonding based on metallic nanowire surface fasteners. Nanotechnology 2012, 23, 365202. (29) Wang, P.; Ju, Y.; Chen, M.; Hosoi, A.; Song, Y.; Iwasaki, Y. Room-temperature bonding technique based on copper nanowire surface fastener. Appl. Phys. Express 2013, 6, 035001. (30) Taberna, P. L.; Mitra, S.; Poizot, P.; Simon, P.; Tarascon, J. M. High rate capabilities Fe3O4-based Cu nano-architectured electrodes for lithium-ion battery applications. Nat. Mater. 2006, 5, 567−573. (31) Puippe, J.-C.; Leaman, F. In Theory and Practice of Pulse Plating; American Electroplaters and Surface Finishers Society: Orland, FL, 1986. (32) Molares, M. E. T.; Buschmann, V.; Drobrev, D.; Neumann, R.; Scholz, R.; Schuchert, I. U.; Vetter, J. Single-crystalline copper nanowires produced by electrochemical deposition in polymeric ion track membranes. Adv. Mater. 2001, 13, 62−65. (33) Elizondo-Decanini, J. M.; Dudley, E. Pulsed high-voltage breakdown of thin-film parylene C. IEEE Trans. Plasma Sci. 2011, 39, 3162−3167. (34) Kahouli, A. Effect of film thickness on structural, morphology, dielectric and electrical properties of Parylene C films. J. Appl. Phys. 2012, 112, 064103. (35) Tadmor, R. The London−van der Waals interaction energy between objects of various geometries. J. Phys.: Condens. Matter. 2001, 13, 195−202. (36) Leckband, D.; Israelachvili, J. Intermolecular forces in biology. Q. Rev. Biophys. 2001, 34, 105−267. (37) Visser, J. On Hamaker constants: A comparison between hamaker constants and Lifshitz−van der Waals constants. Adv. Colloid Interface Sci. 1972, 3, 331−363. (38) Mo, Y. F.; Turner, K. T.; Szlufarska, I. Friction laws at the nanoscale. Nature 2009, 457, 1116−1119.

(39) Goda, T.; Konno, T.; Takai, M.; Ishihara, K. Photoinduced phospholipid polymer grafting on parylene film: Advanced lubrication and antibiofouling properties. Colloids Surf. B. 2007, 54, 67−73. (40) Kim, S.; Sitti, M.; Hui, C. Y.; Long, R.; Jagota, A. Effect of backing layer thickness on adhesion of single-level elastomer fiber arrays. Appl. Phys. Lett. 2007, 91, 161905. (41) Tokuno, T.; Nogi, M.; Jiu, J.; Sugahara, T.; Suganuma, K. Transparent electrodes fabricated via self-assembly of silver nanowire using a bubble template. Langmuir 2012, 28, 9298−9302. (42) Zhu, Y.; Hill, C. M.; Pan, S. Reductive-oxidation electrogenerated chemiluminescence (ECL) generation at a transparent silver nanowire electrode. Langmuir 2011, 27, 3121−3127. (43) Wu, H.; Hu, L.; Rowell, M. W.; Kong, D.; Cha, J. J.; McDonough, J. R.; Zhu, J.; Yang, Y.; McGehee, M. D.; Cui, Y. Electrospun metal nanofiber webs as high-performance transparent electrode. Nano Lett. 2010, 10, 4242−4248.

13916

dx.doi.org/10.1021/la402475f | Langmuir 2013, 29, 13909−13916