Laser Shock-Based Platform for Controllable Forming of Nanowires

Here we report a scalable strategy to conduct cutting, bending, and periodic straining of NWs by making use of laser shock pressure. Three-dimensional...
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Laser Shock-Based Platform for Controllable Forming of Nanowires Ji Li, Yiliang Liao, Sergey Suslov, and Gary J. Cheng* Birck Nanotechnology Center and School of Industrial Engineering, Purdue University, West Lafayette, Indiana 47906, United States ABSTRACT: One-dimensional nanomaterials have attracted a great deal of research interest in the past few decades due to their unique mechanical, electrical, and optical properties. Changing the shape of nanowires (NWs) is both challenging and crucial to change the property and open wide functions of NWs, such as strain engineering, electronic transport, mechanical properties, band structure, and quantum properties, etc. Here we report a scalable strategy to conduct cutting, bending, and periodic straining of NWs by making use of laser shock pressure. Three-dimensional shaping of silver NWs is demonstrated, during which the Ag NWs exhibit very good ductility (strain-to-failure reaches 110%). Meanwhile, the high electrical conductivity of Ag NWs could retain well under controlled laser shock pressure. The microstructure observation indicates that the main deformation mechanism in Ag NWs under dynamic loading is formation of twinning and stacking fault, while dislocation motion and pile-up is less obvious. This method could be applied to semiconductor NWs as well. KEYWORDS: laser shock, scalable 3D shaping, nanowires, formability, mechanical property, electrical property

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ith the ever-growing research development in NW synthesis,1,2 more and more research interest have been focused on postprocessing of NWs, such as bending,3,4 buckling,5,6 and cutting NWs.7,8 Inducing controllable forming of NWs provides important means for fundamental study in strain engineering, electronic transport, mechanical properties, band structure, quantum properties, and so forth. As interconnects or other basic building blocks in the fields of nanoelectronics, nanoactuators, nanoresonators, and nanosensors,9,10 NWs with tunable shape are able to accommodate to various surface structure requirement, such as flexible electronics. In addition, studies have also shown that mechanical forming in NWs may change the electrical, optical, and chemical properties of the NWs,5,11,12 providing many opportunities for developing new generation of NW-based devices. However, despite the importance of controlling the size and shape of NWs, there are very limited methods for scalable shaping of NWs. Although forming and machining are common processes to change bulk metallic component into desired shape, it is still a great challenge in nano world. Currently, atomic force microscopy (AFM) tips are often used to apply forces on the NWs in order to measure their mechanical properties. A single AFM tip has been used to push NWs lying over a trench,13 and two AFM tips can be used to hold and bend NWs.3 However, an AFM tip cannot be used to manipulate NWs practically since only one NW can be treated at a time and it is not feasible to generate complicated shape changes. Therefore, there is a need to develop scalable nanomanufacturing processes that can effectively form and machine large quantity of NWs to open wide applications of NWs. Here we present a way to flexibly shape NWs in a scalable and controllable manner. The schematic setup is shown in © 2012 American Chemical Society

Figure 1. The method using laser-induced shock pressure to generate 3D shape changes in thin films has been studied previously.14−17 A nanosecond pulsed laser is used as the energy source to irradiate the target, which sequentially consists of a confinement layer (glass), an ablative coating layer (graphite), an ultrathin metal foil, an elastomeric material layer, aligned NWs, and a silicon nanomold (Figure 1a). The silver NWs that are coated on the lower side of the elastomeric material or the surface of the mold from ethanol suspension by dip coating with a larger portion of the NWs tend to align in the direction of pulling. When the laser pulse transmits through the confinement and ablates the ablative coating, the ablative coating quickly vaporizes and ionizes into plasma (Figure 1b). The expansion of the plasma is confined and bounced off by the confining media, generating a strong shock wave, which provides a sufficient momentum to shape the metal foil and thus the underlying NWs onto the mold. It is important that the metal foil flows into the mold cavities because it provides conformal contact between metal foil and NWs, ensuring effective pressure to transfer from the metal foil to the NWs. In order to prevent damage of the NWs, a layer of elastomeric material is coated on the metal foil, which absorbs the shock energy. By adjusting process conditions, such as laser intensity, substrate (mold) material, and mold shape, four different types of forming results (conformal shaping, uniform bending, cutting, and lateral compression) have been successfully achieved on silver NWs. The flexible shaping ability makes this method an effective tool to fulfill different requirement of configuration of NWs in different applications. This technique provides a new way for scalable processing of NWs in that Received: March 30, 2012 Revised: May 9, 2012 Published: May 17, 2012 3224

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Figure 1. Laser shock based controllable forming of NWs. (a) Schematic of the forming unit consisting of different layers. (b) Process of the laser shock forming. (c) SEM images of an original silver NW and silicon mold with array of channels. (d) Schematic process of conformal shaping NWs. (e) SEM images of conformal shaped silver NW. (f) Schematic process of uniform bending NWs. (g) SEM images of bended Ge NWs. (h) Schematic process of cutting NWs. (i) SEM images of cut silver NW. (j) Schematic process of lateral compressing NWs. (k) SEM images of compressed silver NW.

shock pressure is determined by the laser intensity, shock impedance of the confining media and ablative coating, absorption coefficient of laser energy in plasma generation, and plasma layer thickness. Since the laser pulse width, confining media, and ablative coating are fixed in the experiment, the laser shock pressure is controlled by laser intensity. The laser intensity used in this study ranges from 0.07 to 0.2 GW/cm2, and the calculated laser shock pressure ranges from 400 to 1000 MPa. Note that the laser shock pressure is that applied on the metal foil. The magnitude of pressure applied on the NWs could be reduced by the elastomeric layer coated on the metal foil, which absorbs the shock energy. The laser intensity of 0.07 to 0.2 GW/cm2 could be achieved on 1 cm2 area with pulse energy of 0.35 to 1 J and pulse duration of 5 ns (Nd/YAG laser, model: Continuum Surelite III). In order to apply uniform pressure on large areas, a beam diffuser could be used to flatten the Gaussian beam profile. With a motion controlled table or a beam scanner and laser pulse frequency of 20 Hz, this process could used to apply large scale shaping of nanowires on a 6 in. wafer within a minute. This method is also flexible in forming types. By adjusting the mold shape, size, and material used, four types of forming results have been achieved on an original straight and smooth silver NW (Figure 1c). Figure 1f−k depicts the schematic

nanoscale forming could be successfully induced in a large quantity of NWs lying over a macroscale area (mm) in a onestep operation. Silver NW was chosen in this study because it exhibits the highest electrical and thermal conductivities among all metals, which makes it a perfect candidate for interconnects and active components in fabricating micro/nano devices. Meanwhile, silver is one of the most ductile metals, so it is suitable for mechanical forming and shape change. The silver NWs used here were synthesized using vapor−liquid−solid method. They are single crystals with a diameter around 100 nm. Controllable forming of NWs also enables fundamental study of mechanical response and structure evolution of NWs under dynamic shock loading, especially at the plastic range, which was majorly studied by numerical simulation due to the lack of experimental approaches. MD simulation has been used widely to study the deformation behavior of NWs in both elastic and plastic region,18,19 but the results are rarely verified because of ultrahigh strain rate (normally >107 s−1) in the simulation. The underlying mechanisms for the observed deformation behavior of silver NWs will be discussed. In this study, the shaping of NWs can be controllable by adjusting the applied pressure induced by laser shock. According to the Fabbro’s model,20 the magnitude of laser 3225

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Figure 2. Conformal shaping of NWs. (a) SEM image of a Ag NW half-shaped into quasi-rectangular waves with the smallest feature length (convex part) ∼ 80 nm. (b) AFM image (inlet) and height information along the NW shown in panel a. (c) Schematic of the NW being shaped by the uniform applied pressure. (d) SEM image of a fractured wavy Ag NW, showing the preservation of the bending geometrical shape after breaking. (e) The width and strain variation along the NW shown in panel d from point a to point b, depicting the compression effect of the laser shock pressure at bar region (B). Necking is also observed, indicating ductile fracture of the Ag NW. The maximum local strain before fracture is found to be 1.1.

strain can be estimated by ε = ln(wd/w0), where wd and w0 are the measured width of NWs before and after lateral compression. Detailed analysis on conformal shaping of silver NWs are given in Figure 2. The NWs can be formed into either periodic or aperiodic wavy structure. Figure 2a shows an SEM image of a NW half-shaped into quasi-rectangular waves, and AFM is used to measure the height profile along the deformed NW, as shown in Figure 2b. It can be seen that the shape of the mold is well taken by the NWs with the feature size as small as 80 nm and depth of 60 nm. However, with a rigid mold used for good shape copy in conformal shaping process, the compression effect of the shock pressure on the part of NW located at the bar position of the mold is significant, preventing the NWs to be formed into high aspect ratio features, as illustrated in Figure 2c,d. The compression effect is especially severe when high laser intensity is used. Applying elastomeric material layer on top of NWs effectively reduces the compression effect. As shown in Figure 3e, a layer of 10 nm polyvinylpyrrolidone (PVP) coating increases the laser shock pressure by 100 MPa to compress a silver NW to the same compressive strain, and a layer of 1 μm polyvinyl alcohol (PVA) coating increases 400 MPa. By further adopting polymer mold, the compression effect can be almost eliminated, which becomes a bending process. There is no obvious compression observed in bended NW, and the NWs are uniformly elongated in bending process. The silver NWs also exhibited superplasticity during the high strain rate forming process. A NW deformed to its forming limit in conformal shaping is shown in Figure 2d. Fracture takes place when the deformation exceeds the forming limit of the NW. The formed shape of NWs preserves after crack initiation. Necking takes place before crack initiation, indicating a ductile

processes and typical forming results of the different forming types, which are conformal shaping, uniform bending, cutting, and lateral compression, respectively. Among these four cases, conformal shaping is a basic type, which is obtained by using a rigid mold with low aspect ratio features (Figure 1d). It can be seen that the silver NWs go into the mold cavity, conformably taking the shape of the mold (Figure 1e). The wavelength of the nanowires after shaping is as small as 80 nm. When the mold cavities have high aspect ratio and sharp edges (no fillet), instant fracture of the NW occurs due to local concentration of severe plastic deformation (Figure 1h). The strain concentration is caused by both shearing and tension. As a result, the NW breaks into shorter segments with the new length defined by the dimension of the mold cavities (Figure 1i). The advantage of this cutting method is that a large amount of NWs can be cut at the same time in one step operation, which is superior to other NW cutting methods, such as electrical beam cutting. Additionally, both metallic NWs and semiconductor NWs can be cut in this way. The third type is bending of NWs, which is obtained by using an elastomeric polymer mold (Figure 1f). Because of the high elasticity of the polymer material, the mold is bended by the shock pressure during the forming process. Accordingly, the NW deforms with the bended surface of the polymer mold, resulting in bending (Figure 1g). In addition, with the elastomeric material playing the role of “cushion” layer that absorbs the shock energy, the deformation is uniform along the bended NWs compared to conformal shaped ones. If there is no cavity on the substrate but only a flat surface, lateral compression of the NWs can be obtained (Figure 1j). The longitude deformation is negligible in lateral compression of NWs, thus, it is a plain strain condition. The compressive 3226

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Figure 3. Lateral compression of NWs. (a,b) SEM images of an identical Ag NW before and after lateral compression. The inlets show the cross section of the NW (tilt 52°). The diameter of the NW is measured as 108 and 253 nm before and after compression, and the compressive strain is 0.85. (c) Typical SEM image of a crushed Ag NW. (d) Processing plot of the lateral compression process with a schematic diagram. Two critical pressure levels, ∼500 and ∼750 MPa, divide the plot into three areas, which are no deformation, compression, and crushing of NWs, respectively. (e) The relationship between shock pressure and the compression strain under different polymer coating conditions: without polymer, with 10 nm PVP, and with 1 μm PVA. The shock absorbing material is effective in reducing the compression effect.

pressure on bar region. The effective strain has the same trend, which is much higher at the bar region, suggesting obvious contribution of compression in the effective strain, which lowers the elongation limit of the NWs. Thus, reducing compression effect will yield more uniform deformation distribution and increases the attainable feature aspect ratio. A dent is observed on the width variation curve, which corresponds to the necking position on the NW. The local effective strain at the necking position suddenly increases to as high as 1.05, indicating a very good ductility of the silver NWs. The results on lateral compression of NWs are shown in Figure 3. Figure 3a,b shows the SEM images of a NW before and after laser pressure compression. The inlets show the

fracture type of silver NWs, which is in consistent with bulk silver. For unit feature in the periodic wavy NWs, the elongation can be estimated. It is assumed that the NW is pinned at the bar center position of the mold by the strong shock pressure, so the maximum elongation is estimated as 30% for conformal shaping. The classic equation εl = ln(ld/l0) = ln(A0/Ad) can be used to calculate the tensile strain, and the effective strain is calculated from the von Mises criteria εeff = (2/3(ε12 + ε22 + ε32)1/2. The width variation and corresponding effective strain from point a to b in Figure 2d are measured, calculated, and plotted in Figure 2e. It can be seen that the width level of the NW in bar region (B) and trench region (T) are different, which is caused by compression effect of the 3227

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Figure 4. Microstructure characterization and electrical property measurement of deformed Ag NW. (a) The relationship between the compressive strain and laser shock pressure. (b) TEM image of the original NW, showing a quite uniform single crystal structure. (c) TEM image of NW compressed by pressure corresponding to position II indicated by red arrows in a. A lot of contrasts are observed, which are mainly caused by densely spaced deformation twins and dislocation pile-ups. (d) The selected diffraction pattern (SADP) of original NW (upper) shown in panel b from zone axis [110], and SADP of compressed Ag nanowire (lower) shown in panel c, indicating twining structures. (e,f) High-resolution TEM images of the area in red square and green square on the compressed NW shown in panel c, respectively. (g) Measured I−V curves of original Ag NWs and deformed Ag NWs shocked by pressure corresponding to position II indicated by red arrows in panel a. The inlet is the SEM image of a deformed Ag NW device.

NW deformed by a high pressure of 680 MPa, marked as position II in Figure 4a, and the corresponding compressive strain is almost 0.8. Both are observed from the zone axis [110]. The corresponding selected area diffraction patterns (SADP) are shown in Figure 4d. It can be seen that the as-received silver NW is quite uniform and defect free with a single crystal structure. After compression, high density of deformation twins were observed in NWs. Meanwhile, pileups of dislocations also appear in the inner part of the NW. Figure 4e,f shows highresolution TEM images of certain areas of the deformed NW. Different from plastically deformed bulk material, the dislocation density in the NWs is relatively low. It was observed that the main deformation mechanism of silver NWs under dynamic shock loading is controlled by twinning and stacking faults formation. This observation is consistent with previous MD simulation results, which found that partial dislocation slip and twin are the major plastic deformation modes for FCC metals NWs when strain rate is below 1010 s−1 but rarely have been validated. Our results provide direct experimental evidence confirming these simulation results. The dense deformation twins start from the surface of the NW and goes into the interpart, indicating that the partial dislocation first nucleates at the surface and slips afterward. The reason for surface to be the favorable nucleation position is the lack of defect in NWs due to their small radial dimension. The boundaries of severely deformed NWs are no longer straight and uniform, which is caused by formation of partial dislocations. The formation of closed spaced deformation twins explains the good ductility of the Ag NWs, as the twin boundaries can be available as dislocation sources with further straining.

corresponding cross sections cut by focused ion beam (FIB), and the samples are tilted 52°. It can be seen that the wire sustained a dramatic shape change from a round cross section to a flat ribbon shape, and the compressive strain is high (∼0.85). The relationship between laser shock pressure and compressive strain was calibrated. The plastic compression starts at about 500 MPa, which is about 10 times larger than the yield strength of bulk silver. It is in consistent with the increasing yield strength with the decreasing size. The main reason is that the internal defect becomes less and less with the decreasing size. And a sufficient energy is needed to activate and nucleate dislocations. When the shock pressure exceeds 500 MPa, NWs start to be compressed, and the compressive strain is increasing with the pressure until about 750 MPa. When the shock pressure is higher than 750 MPa, crushing of the nanowires is observed. As seen in Figure 3c, the NW is crushed into a collection of nanopieces. Note that the critical pressure is not a precise value but has a range, because it is related to the specific diameter of the NW being processed, which has a distribution around 100 nm. The ultimate compressive strain obtained without sign of crushing is estimated to be ∼1.1, and thus the effective strain is nearly 1.4. Therefore, the silver NW shows very good ductility in both bending and lateral compression. Plastic deformation is always accompanied by the change of microstructure in the material, due to dislocation motion and multiplication. Here, the microstructure evolution of the silver NW laterally compressed by shock pressure is directly observed by TEM, as seen in Figure 4b−f. Figure 4b shows the TEM image of an original Ag NW. When the pressure is less than 500 MPa, as indicated by red arrow in Figure 4a, no deformation is induced in the Ag NWs. Figure 4c shows the TEM image of a 3228

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In a summary, laser shock-based technique for controlled forming of nanowires provides a potential pathway for massive forming and machining of metallic nanowires, and it has the following advantages: (1) the process provides a unique scalable nanomanufacturing for roll-to-roll processing of NWs at room temperature and atmospheric pressure with no vacuum system and no lithography; (2) one step fast process in which the duration of laser pulse is only tenths of nanosecond; (3) tunable in which the shape/size of the nanowires could be manipulated by the laser and mold conditions; and (4) flexible in which it has been used to deform nanowires of various materials, including semiconductor NWs such as silicon, germanium NWs. Methods. Setup of Laser Dynamic Forming. A short pulsed Q-switch Nd:YAG laser (Continuum Surelite III) is used as the energy source. The laser beam has a Gaussian distribution and the pulse width is 10 ns. A focus lens is used to control the beam size. The beam diameter used is 4 mm, which is calibrated by a photosensitive paper (Kodak Linagraph, type: 1895). The laser power is measured by an optical power meter (Newport, type: 1916c). Glass slide is used as the confining media, and aerosol graphite painting (Asbury Carbons, U.S.A.) is sprayed on 4 μm thick aluminum foil (Lebow Company Inc., Bellevue, WA) as the ablative coating. The thickness of the ablative coating is in the range from 1 to 10 μm. An X−Y stage is used to move the processing stage. Coating of Silver NWs. Silver NWs (Blue Nano, NC, U.S.A.) are first dispersed in ethanol by sonication in ultrasonic cleaner (VWR-B1500A_DTH). Then a dip coater (EQHWTL_01_A, MTI Corporation) is used to coat the NWs on 4 μm thick free-standing aluminum foil or silicon mold. The coating speed used is 40 mm/min. PVP (Sigma-Alderich) or PVA (Sigma-Alderich) is dip coated on the Al foil in the same way as the cushion layer. Preparation of Silicon Mold. Two methods are used prepare mold by cutting patterns on silicon wafers. The first one is FIB milling performed by FEI Nova 200 Nano-Lab DualBeam TMSEM/FIB. The second method is electron beam lithography using Leica VB-6 ultrahigh resolution, extra-wide field electron beam lithography tool. Poly(methyl methacrylate) is used as the photo resist and is developed away after exposure. The rest of the photo resist is left on the surface of the silicon wafer, playing the role of cushion for shock absorbing in LDF. Characterization of Deformed NWs. The SEM images of NWs were recorded on a Hitachi S-4800 field-emission scanning electron microscope. The AFM images and height profile of the NWs were measured by a DI Dimension 3100 atomic force microscope. The microstructure of the original and deformed NWs was examined using a FEI Tecnai transmission electron microscope. The electrical measurements were carried out using a HP4156 parameter analyzer. The electrodes used are platinum, which were deposited on the NWs by FIB using FEI Nova 200 Nano-Lab DualBeam TMSEM/FIB.

Two reasons are responsible for the observed deformation twins in the compressed Ag NWs. The first one is small grain size. For bulk FCC metal, the critical twinning stress is much higher than the slip stress. With the decreasing of grain size, the energy needed to form twins and stacking faults gradually becomes closer to the energy needed to nucleate a perfect dislocation, so the deformation mechanism controlled by normal slip transforms to that controlled by partial dislocation activity. The critical grain size can be estimated by21 Dc =

2αμ(bN − bp)bp γ

(1)

where μ is shear modulus (∼30 GPa for silver), γ is the stacking fault energy [∼23 mJ/m2 for silver22], α reflects the character of the dislocation, bN and bP are the magnitudes of the Burgers vectors of the perfect dislocation and the Shockley partial dislocation, respectively. Taking α = 1, the estimated Dc is about 60 nm for silver. However, the size of silver NW used in the experiment (∼100 nm) is quite large compare to the estimated critical size for twinning to be dominant. Thus, besides small size, there is another important reason for twinning and stacking faults formation in the lateral compressed NW, which is the ultrahigh strain rate generated by laser shock pressure. The dislocation-slip process is suppressed when the strain rate is high, assisting the formation of deformation twins. Besides deformation twins, other local deformation structures, such as dislocations, are also observed, which is reasonable because of inhomogeneous deformation. In addition, although the radial dimension of NW is very small, which is not favorable for formation of full dislocation, the longitude dimension is quite large, providing enough space for dislocations to move and pile up. Thus, in the inner part of the NW, dislocation walls are seen. This is in consistent with previous experimental reports that dislocations are still operative in NWs with a diameter down to 40 nm.13 Thus, both twining and dislocation are active in the deformation process of silver NWs with a diameter from 50 to 100 nm under dynamic shock loading. With the decreasing of NW diameter, twinning will be more and more favorable. Also, it can be predicted that when a critical diameter is reached, twinning will become the dominant mechanism for the NW deformation, even at a low strain rate. The electrical conductivity of the Ag NW before and after laser forming was also studied. Two terminal electrical measurements were made, as shown in the Figure 4g. The shock pressure applied to deform the NW is about 680 MPa, given as position II in Figure 4a. It can be seen that the two resultant I−V curves are almost identical with obvious linearity and a stable ohmic resistance of 137 Ω for the original Ag NWs and 135 Ω for the deformed Ag NWs. The negligible change in the I−V curve indicates that the severe plastic deformation induced by laser forming does not diminish the good conductivity of Ag NW. The preservation of low resistivity of strained Ag NWs could be explained by the deformation mechanism in the process, which is mainly controlled by twinning, as discussed. Twinning boundary is known as a coherent boundary, which has small influence on electrical resistivity. Therefore, the electrical property of Ag NW retains very well after the high strain rate laser forming treatment. Note that the resistivity value is higher than the bulk value because of surface scattering of metal NWs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.J.C.). Notes

The authors declare no competing financial interest. 3229

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(21) Chen, M.; Kevin, E. M.; Hemker, J.; Sheng, H. Y. wang, X. Cheng, Deformation twinning in nanocrystalline aluminum. Science 2003, 300, 1275−1277. (22) Ruff, A. W., Jr.; Ives, L. K. Dislocation node determinations of the stacking fault energy in silver-tin alloys. Acta Metall. 1967, 15, 189−198.

ACKNOWLEDGMENTS The authors want to thank support from US NSF (Grant No. CMMI 0928752) and CAREER Award (Grant No. CMMI0547636) through program of Materials Processing & Manufacturing.

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REFERENCES

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