NANO LETTERS
Nanoindentation of Silver Nanowires Xiaodong Li,*,† Hongsheng Gao,† Catherine J. Murphy,‡ and K. K. Caswell‡
2003 Vol. 3, No. 11 1495-1498
Department of Mechanical Engineering, UniVersity of South Carolina, 300 Main Street, Columbia, South Carolina 29208, and Department of Chemistry and Biochemistry, UniVersity of South Carolina, 631 Sumter Street, Columbia, South Carolina 29208 Received July 16, 2003; Revised Manuscript Received September 24, 2003
ABSTRACT The hardness and elastic modulus of a silver nanowire was measured using a nanoindenter. It was found that the silver nanowire has comparable hardness and elastic modulus to bulk silver. An array of nanoscale indents was successfully made on the wire by directly indenting the wire. The shape and size of the indents are controllable. The nanoindentation approach permits the direct machining of nanowires without the complications of conventional lithography.
Micro/nanodevices need conductors to provide power as well as electrical/magnetic signals to make them functional. Electrical/thermal conductors are becoming necessary elements in today’s advanced micro/nanodevices.1-3 Silver is an attractive material because it exhibits the highest electrical and thermal conductivities among all metals.4-7 Silver has been extensively exploited in a variety of applications that range from catalysis, through electronics, to photonics and photography. As a result, the synthesis and characterization of silver nanowires have recently attracted attention from a broad range of researchers. In addition to their potential use as interconnects or active components in fabricating micro/ nanodevices, silver nanowires may also provide an ideal system for experimentally probing physical phenomena such as quantized conductance and localization effects.5 Although silver nanowires have been successfully synthesized by various techniques, their mechanical properties have not been explored yet. Mechanical and structural aspects are of critical importance in integrating nanoscale building blocks into functional micro/nanodevices. Recent studies have revealed that material properties are size-dependent.1,8-10 However, current software developed for designing micro/ nanoscale devices is based on bulk material properties without considering the size-dependent phenomenon. This limits the further development and application of micro/ nanodevices. One of the challenges in the commercialization of micro/ nanodevices is the machining of nanostructures.2,3 Can one machine a nanowire as easily as one can machine a metal bar? To date, nanomachining a nanowire with nanometer accuracy has not been achieved. A technical breakthrough * Corresponding author. E-mail:
[email protected]. † Department of Mechanical Engineering. ‡ Department of Chemistry and Biochemistry. 10.1021/nl034525b CCC: $25.00 Published on Web 10/10/2003
© 2003 American Chemical Society
in generating nanometer structures, such as nanoslots and nanoindents on a nanowire, is greatly needed for the integration and manufacture of functional micro/nanodevices. The extremely small dimensions of silver nanowiress diameters of a few tens of nanometers and lengths of a few micrometerssimpose a tremendous challenge for the experimental study of mechanical properties and machining. The objectives of the research were to measure the mechanical properties of silver nanowires directly and to explore the possibility of machining the wires using a nanoindenter. In the present study, the combination of an atomic force microscope (AFM) and a nanoindenter was used to visualize a single silver nanowire and then in situ indent the wire with the precise placement of the indenter tip on the wire. After the indentation, the same indenter tip was used to image the indentation impression on the wire. The shortcoming of using an AFM to perform indentation tests is that the AFM tip cannot be perpendicular to the sample surface, thereby causing slip and friction between the AFM tip and the sample surface during indentation. The slip friction force makes it impossible for the AFM to measure the indentation load and displacement accurately on the nanoscale. In addition, high indentation load cannot be reached because of limitations of AFM cantilever stiffness. Compared with AFM cantilever beam indentation tests, the benefits of using a nanoindenter in conjunction with an AFM are the more effective exploration of the sample, improved load and displacement resolutions, the achievement of high loading, and the ability to observe the material’s response to indentation in near-real time. We report, for the first time to our knowledge, the hardness and elastic modulus of a silver nanowire. We also used the nanoindenter to create an array of nanoscale indents on a silver nanowire and to cut the wire with nanometer accuracy.
Silver nanowires were synthesized using a combination of two silver solutions, A and B, prepared in glassware washed thoroughly with soap (Alconox), deionized water, and aqua regia.4 Solution A consisted of 100 mL of deionized H2O, 1.5-2 µL of 1 M NaOH (Fisher), and 40 µL of 0.1 M AgNO3 (Sigma). This solution, in an Erlenmeyer flask, was brought to a boil with rapid stirring. Quickly added to A was 5 mL of 0.01 M trisodium citrate (Fisher), and then the solution was allowed to boil for an additional 10 min. Concomitantly, solution B was made by mixing 150 mL of deionized H2O and 1.5-2 µL of 1 M NaOH with 20 µL of 0.1 M AgNO3 and bringing this to a boil. Solution B was added to A, the neck of the flask was covered with an inverted 50-mL beaker, and the mixture was allowed to boil for 60 min while stirring rapidly. When the mixture evaporated to ∼75 mL, additional quantities (∼75 mL) of boiling deionized water were added (this usually occurred at ∼30 min). The final volume was typically ∼75-100 mL, and the final pH was ∼7.0-7.1. For detailed information about the formation mechanisms of silver nanowires, please see ref 4. A drop of solution with the silver nanowires was put on a glass slide for the AFM and nanoindentation tests. A Hysitron Troboscope nanoindenter in conjunction with a Veeco Dimension 3100 AFM was used to perform imaging and nanoindentation tests. The nanoindenter monitors and records the load and displacement of the three-sided pyramidal diamond (Berkovich) indenter during indentation with a force resolution of about 1 nN and a displacement resolution of about 0.2 nm.10 The indenter tip was used to image and locate a silver nanowire and then in situ indent the wire with the same tip. The indentation impression was also imaged with the same tip. Post-test imaging provides the ability to verify that the test was performed in the anticipated location, which maximizes the reliability of the data and aids in the explanation of unexpected test results. The hardness and elastic modulus were calculated from the load-displacement data obtained by nanoindentation. Nanoindentation hardness is defined as the indentation load divided by the projected contact area of the indentation. It is the mean pressure that a material will support under load. From the load-displacement curve, hardness can be obtained at the peak load as H)
Pmax A
(1)
where Pmax is the peak load and A is the projected contact area. The nanoindentation elastic modulus was calculated using the Oliver-Pharr data analysis procedure11 beginning by fitting the unloading curve to a power-law relation. The unloading stiffness can be obtained from the slope of the initial portion of the unloading curve, S ) dP/dh. On the basis of relationships developed by Sneddon12 for the indentation of an elastic half space by any punch that can be described as a solid of revolution of a smooth function, a geometry-independent relation involving contact stiffness, 1496
contact area, and elastic modulus can be derived as follows:
xAπE
S ) 2β
(2)
r
where β is a constant that depends on the geometry of the indenter (β ) 1.034 for a Berkovich indenter)13 and Er is the reduced elastic modulus that accounts for the fact that elastic deformation occurs in both the sample and the indenter. Er is given by 1 1 - ν2 1 - νi + ) Er E Ei
2
(3)
where E and ν are the elastic modulus and Poisson’s ratio for the sample and Ei and νi are the same quantities for the indenter, respectively. For diamond, Ei ) 1141 GPa and νi) 0.07.11 The representative AFM images of a silver nanowire are shown in Figure 1. The wire is about 4 µm long and 42 nm in diameter. The wire surface is smooth without any nanoscale features. Electron diffraction4 shows that the silver nanowire is not a single crystal but twinned. The adhesion forces between the wires and the glass slide are strong enough to prevent the wire from moving/rolling on the glass slide. We found that we could reproducibly image the wires in contact mode. The wires were not dragged away by the tip, indicating large adhesion forces. Such high adhesion forces were also reported for carbon nanotubes.14 A Berkovich diamond nanoindenter tip was used to image and locate a single nanowire and then in situ position the indent tip on the wire to perform an indentation test. An array of nanoscale indentations at different indentation loads was made. Figure 2 shows the AFM images of the indents on the wire and a representative load-displacement curve. The peak nanoindentation depth can be as low as 15 nm, which is about 30% of the wire diameter. It is generally accepted that the depth of indentation should never exceed 30% of the wire diameter (film thickness).10 Therefore, the substrate effect on the measurement of the hardness and elastic modulus of the wire can be ignored. The indentation impressions were imaged immediately after the indentation tests using the same tip. The indentation projected areas obtained by the AFM were used to calibrate the hardness and elastic modulus values. Such calibration can also eliminate the wire curvature effect on the hardness and elastic modulus measurement. A pop-in mark or step was found at the indentation load of in the loading curve. Before the popin mark, the loading curve can be fitted by the Hertzian elastic contact mode. The pop-in mark indicates plastic deformation associated with dislocation nucleation and motion.15 The existence of dislocations makes the indentation more difficult. After the indentation projected area calibration, the hardness and elastic modulus values of the silver nanowires were measured to be about 0.87 ( 4 and 88 ( 5 GPa, respectively. The nanoindentation hardness and elastic modulus are in good agreement with the nanoindentation Nano Lett., Vol. 3, No. 11, 2003
Figure 1. AFM images of a silver nanowire. (a) 2D AFM image, (b) 3D AFM image, and (c) height profile of the wire.
results of the bulk silver single crystal.15-17 The silver nanowires exhibit size-dependent hardness; the nanoscale hardness of silver nanowires is about 2 times higher than the macro/microscale indentation hardness of the bulk single crystal. Such size-dependent hardness was also found in the bulk silver single crystal for indentation depths of less than 1 µm.16,17 The hardness obtained from large indentations with indentation depths larger than 1 µm is about 0.4 GPa. The indentation size effect in silver in the micro/nanometer range is due to the large strain gradients inherent in small indentations that lead to geometrically necessary dislocations.16,17 Although many nanomaterials such as nanoparticles, nanotubes, nanowires, and nanobelts have been developed, the machining and integration of these nanoscale building blocks remain a great challenge. The nanoindenter and AFM have already been used for nanofabrication such as nanoNano Lett., Vol. 3, No. 11, 2003
Figure 2. (a and b) AFM images of indents on a silver nanowire, (c) height profile of an indent on the wire, and (d) a representative nanoindentation load-displacement curve.
lithography, nanowriting, and nanopatterning. In our study, an array of nanoscale indents was successfully made on a single silver nanowire. The depth and size of indents can be 1497
In summary, the hardness and elastic modulus of a silver nanowire were measured using a nanoindenter. The silver nanowire has comparable hardness and elastic modulus values to those of the bulk silver. An array of nanoscale indents was successfully made on the wire by nanoindentation techniques. The nanoindenter is able to cut the silver wire by making a high load indentation at the desired position. The combination of a nanoindenter and an AFM should find more applications in the integration and manufacturing of nanodevices. Acknowledgment. Financial support for this study was provided by the National Science Foundation (contract no. EPS-0296165), the South Carolina Space Grant ConsortiumNASA, and the University of South Carolina NanoCenter. The content of this information does not necessary reflect the position or policy of the Government, and no official endorsement should be inferred. References
Figure 3. AFM images of the cut silver nanowires and indentation impressions. (a) 2D image and (b) 3D image.
controlled by making indentations at different indentation loads. Indents of different shapes can be obtained by using indenters of different geometries such as a cube corner and conical and spherical tips. Such nanoscale indents can be used as cells for molecular electronics and drug delivery, slots for integration of the nanowires into nanodevices, and defects for tailoring the structure and properties of the nanowires. By using a high indentation load, we can cut a nanowire into the length that is needed (Figure 3). This indicates that if a long nanowire is available then we are able to machine the wire as we normally do on the macroscale. The present work widens the application area of nanoindentation and AFM. We believe that the combination of the nanoindenter and AFM makes the nanoscale machining and integration of these building blocks possible.
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(1) Li, X.; Bhushan, B.; Takashima, K.; Baek, C.-W.; Kim, Y.-K. Ultramicroscopy 2003, 97, 481-494. (2) Goddard, W. A.; Brenner, D. W.; Lyshevski, S. E.; Iafrate, G. J. Handbook of Nanoscience, Engineering, and Technology; CRC Press: Boca Raton, FL, 2003. (3) Lyshevski, S. E. MEMS and NEMS Systems, DeVices, and Structures; CRC Press; Boca Raton, FL, 2002. (4) Caswell, K. K.; Bender, C. M.; Murphy, C. J. Nano Lett. 2003, 3, 667-669. (5) Sun, Y.; Yin, Y.; Mayers, B. T.; Herricks, T.; Xia, Y. Chem. Mater. 2002, 14, 4736-4745. (6) Bhattacharyya, S.; Saha, S. K.; Chakravorty, D. Appl. Phys. Lett. 2000, 77, 3770-3772. (7) Liu, S.; Yue, J.; Gedanken, A. AdV. Mater. 2001, 9, 656-658. (8) Li, X.; Bhushan, B. Mater. Charact. 2002, 48, 11-36. (9) Li, X.; Bhushan, B. Surf. Coat. Technol. 2003, 163-164, 503-508. (10) Bhushan, B.; Li, X. Int. Mater. ReV. 2003, 48, 125-164. (11) Oliver, W. C.; Pharr, G. M. J. Mater. Res. 1992, 7, 1564-1583. (12) Sneddon, I. N. Int. J. Eng. Sci. 1965, 3, 47-56. (13) Pharr, G. M. Mater. Sci. Eng., A 1998, 253, 151-159. (14) Salvetat, J. P.; Kulik, A. J.; Bonard, J. M.; Briggs, G. A. D.; Stockli, T.; Metenier, K.; Bonnamy, S.; Beguin, F.; Burnham, N. A.; Forro, L. AdV. Mater. 1999, 11, 161-165. (15) Christopher, D.; Smith, R.; Richter, A. Nanotechnology 2001, 12, 372-383. (16) Ma, Q.; Clarke, D. R. J. Mater. Res. 1995, 10, 853-863. (17) Zhao, M.; Slaughter, W. S.; Li, M.; Mao, S. X. Acta Mater. 2003, 51, 4461-4469.
NL034525B
Nano Lett., Vol. 3, No. 11, 2003
