NANO LETTERS
Nanoindentation of Cu2O Nanocubes Xiaodong Li* and Hongsheng Gao
2004 Vol. 4, No. 10 1903-1907
Department of Mechanical Engineering, UniVersity of South Carolina, Columbia, South Carolina 29208
Catherine J. Murphy and Linfeng Gou Department of Chemistry and Biochemistry, UniVersity of South Carolina, Columbia, South Carolina 29208 Received July 6, 2004; Revised Manuscript Received August 16, 2004
ABSTRACT Nanoindentation tests were performed directly on solid and hollow cuprous oxide (Cu2O) nanocubes. The hardness and elastic modulus of solid Cu2O nanocubes were measured and compared with the values of bulk Cu2O. It is found that the hollow cube top wall acts as a membrane that bends under an indentation load. The Cu2O nanocubes are more ductile rather than brittle. Deformation behavior and fracture mechanics are discussed in conjunction with the structure of the Cu2O nanocube.
Over the past several decades, much research has been focused on cuprous oxide (Cu2O). Cuprous oxide is a p-type semiconductor with unique optical and magnetic properties,1-3 which makes it a promising material in the fields of solar energy conversion, micro/nanoelectronics, magnetic storage devices, catalysis, and biosensors.4-8 Nanostructured Cu2O with morphologies such as wires, tubes, cubes, rods, and thin films has been synthesized by either chemical or physical methods. However, for such highly promising nanoscale building blocks, their mechanical properties have not yet been explored. This limits the further development and application of Cu2O nanostructures and devices. The hardness of bulk cuprous oxide as a mineral is 3.5 to 4 in Mohs hardness. Recent studies have revealed that material properties are size-dependent.9-11 A precise characterization of the mechanical properties of Cu2O nanostructures is required to use them as structural/functional elements in devices. The extremely small dimensions of nanostructures, such as Cu2O nanocubes, impose a tremendous challenge for experimental study of their mechanical properties. In the present study, the combination of an atomic force microscope and a nanoindenter was used to visualize a single Cu2O nanocube, and then in situ indent the cube with precise placement of the indenter tip on the cube. After the indentation, the same indenter tip was used to image the indentation impression on the cube. The shortcomings of using an atomic force microscope (AFM) to perform indentation tests are 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 * Corresponding author. E-mail: www.me.sc.edu/research/nano/. 10.1021/nl048941n CCC: $27.50 Published on Web 09/02/2004
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© 2004 American Chemical Society
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indentation.12 The slip friction force makes it impossible for the AFM to accurately measure the indentation load and displacement at the nanoscale. In addition, high indentation loads cannot be reached because of the limitation of AFM cantilever stiffness. Compared with AFM cantilever beam indentation tests, the benefits of using a nanoindenter in conjunction with an AFM are a more effective exploration of the sample, improved load and displacement resolutions, the achievement of high loading, and the ability to observe 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 Cu2O nanocube. Deformation behavior and fracture mechanics are discussed in conjunction with the structure of the Cu2O nanocube. The Cu2O nanocubes were synthesized by a reductive procedure in aqueous solution, in the presence of surfactant (cetyltrimethylammonium bromide, surfactant, CTAB, or PEG) to control the growth rate and the dimension of the nanocubes. The Cu2O nanocubes produced this way have an average edge length of 200-450 nm, which is tunable with surfactant concentration. After the reaction, the nanocubes were separated from the solution and the surfactant by centrifugation. For detailed information about the formation mechanisms of Cu2O nanocubes, please see refs 6 and 8. 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 displacement resolution of about 0.2 nm.10,11 The indenter tip with a tip radius of 50 nm was used to image and locate a single nanocube and then
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)
Figure 1. Contact areas as a function of contact depths of nanoindentations made on fused quartz.
in situ indent the cube 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 data and aids in explanation of unexpected test results. Hardness and elastic modulus were calculated from the load-displacement data obtained by nanoindentation, based on the Oliver-Pharr method.13
Pmax A
(1)
where A is the projected contact area. The elastic modulus was calculated using the Oliver-Pharr data analysis procedure,13 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. Based on relationships developed by Sneddon14 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, contact area, and elastic modulus can be derived as follows: S ) 2β
xAπE
r
(2)
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 which accounts for the fact that
Figure 2. (a) TEM image, (b) 2D AFM height image, (c) 3D AFM height image, and (d) AFM phase contrast image of Cu2O nanocubes. 1904
Nano Lett., Vol. 4, No. 10, 2004
Figure 3. (a) 2D in situ AFM image and (b) 3D in situ AFM image of an indentation impression made on a Cu2O nanocube, (c) crosssectional height profile of the indentation impression shown in (a) and (b), and (d) the nanoindentation load-displacement curve.
elastic deformation occurs in both the sample and the indenter. Er is given by 1 1 - ν2 1 - νi ) + Er E Ei
The contact depth can be estimated from the loaddisplacement data using
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. For diamond, Ei ) 1141 GPa and νi ) 0.07.13 To calculate hardness, contact stiffness, and elastic modulus from eqs 1-3, the projected contact area A must be determined from the load-displacement curve. The indenter used in practical nanoindentation testing is not ideally sharp (with a tip radius of 50 nm). Therefore, tip geometry calibration or area function calibration is needed. A series of indentations is made on fused quartz at depths of interest. A plot of Ac versus hc can be curve fit according to the following functional form Ac ) 24.56hc2 + C1hc1 + C2hc1/2 + C3hc1/4 + ...... + C8hc1/128 (4) where C1 through C8 are constants. The lead term describes a perfect Berkovich indenter, the others describe deviations from the Berkovich geometry due to blunting of the tip.13 Nano Lett., Vol. 4, No. 10, 2004
Pmax S
hc ) hmax -
(5)
where is a constant that depends on the indenter geometry ( ) 0.75 for a Berkovich indenter) and hmax the displacement at the peak load.13 For detailed information about the data analysis procedure, please review refs 10 and 11. To calibrate the tip radius, we followed a standard calibration procedure.10,11,13 A series of nanoindentation tests were performed on fused quarts. The contact areas as a function of contact depths are shown in Figure 1. A drop of solution with the Cu2O nanocubes was put on a glass slide for the AFM and nanoindentation tests. It should be noted that all indentations were performed at the centers of Cu2O nanocubes. Power X-ray diffraction study showed that the nanocubes synthesized were crystalline cubic cuprous oxide.6 The transmission electron microscope (TEM) and AFM images of Cu2O nanocubes are shown in Figure 2. The Cu2O nanocubes are uniform in size and shape, although a few smaller Cu2O nanocubes were found by TEM. The Cu2O nanocubes tested by nanoindentation in this study have an edge length between 200 and 280 nm. The cube surface is 1905
Figure 4. (a) Indentation load-displacement curves made on a solid Cu2O nanocube and (b) a hollow Cu2O nanocube, respectively.
clean without any contaminating particles. The nanocubes tend to aggregate on the glass slide and the adhesion forces between the cubes and the glass slide are strong enough to prevent the cubes from moving/rolling on the glass slide. We found that we could reproducibly image the cubes in contact mode at a high normal load. The cubes were not dragged away by the tip, indicating large adhesion forces. Such high adhesion forces were also reported for carbon nanotubes and silver nanowires.12,15 A Berkovich diamond nanoindenter tip was used to image and locate a single nanocube, and then in situ position the indent tip at the center of the cube to perform an indentation test. The peak nanoindentation depth used in this study is about 20% of the cube edge length. It is generally accepted that the depth of indentation should never exceed 30% of the particle diameter (film thickness).10,11 Therefore, the substrate effect on measurement of the hardness and elastic modulus of the cube can be ignored. The indentation impressions were imaged immediately after the indentation tests using the same tip. Figure 3 shows the representative nanoindenter tip AFM images, cross-sectional height profile and load-displacement curve of the indent on the cube. It can be seen from Figure 3 that the cubes do not exhibit sharp edges as those shown in Figure 2, but rather show particlelike shape. This is because the AFM images in Figure 3 were
Figure 5. (a) 2D in situ AFM image and (b) 3D in situ AFM image of an indentation impression made on a Cu2O nanocube at a high indentation load; (c) cross-sectional height profile of the indentation impression shown in (a) and (b), and (d) the nanoindentation loaddisplacement curve. 1906
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obtained by scanning the cube using a Berkovich indenter tip with a large included angle of 143° that resulted in a round-up phenomenon of the cube edges in imaging,11 whereas the AFM images in Figure 2 were taken using an AFM tip with a small included angle of 35°. However, this does not affect the measurement of the residual indentation depth. The residual indentation depth is about 9 nm, as shown in Figure 3c, which is in good agreement with the residual indentation depth measured by the load-displacement curve (Figure 3d). Pile-up (the indented material around the indenter above its original surface10) is found around the indentation impression, indicating the ductile nature of the material. When pile up occurs, calibration of projected contact area is needed. In our study, we imaged the indentation impressions in-situ to obtain the projected contact area. This enabled us to obtain accurate hardness and elastic modulus values. It should be noted that the solid cube and the hollow cube produce different indentation load-displacement curves. In addition to the penetration of the indenter tip into the material, the hollow cube top wall acts as a membrane that is bent under indentation load. Two representative loaddisplacement curves for the solid cube and the hollow cube are shown in Figure 4 (a) and (b), respectively. The solid cube exhibits a smaller indentation displacement while the hollow cube exhibits a larger indentation displacement. The load-displacement curves are smooth without any pop-in marks or discontinuities. This indicates that no cracking occurred during indentation. The indentations on the solid cubes were used to calculate the hardness and elastic modulus that are 0.61 ( 0.2 GPa and 82 ( 12 GPa, respectively, obtained from ten nanoindentation tests at peak indentation loads ranging from 24 µN to 44 µN, corresponding to peak indentation depths from 33 to 48 nm made on the Cu2O nanocubes whose edges vary between 200 and 280 nm. The hardness of Cu2O nanocubes is comparable to that of bulk cuprous oxide mineral (3.5-4 Mohs hardness). It is well known that bulk cuprous oxide is brittle in nature. It is found that the Cu2O nanocubes are more ductile rather than brittle. The Cu2O nanocubes were plastically deformed at higher indentation loads without cracking compared to the load that bulk Cu2O can withstand. Figure 5 shows the
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representative nanoindenter tip AFM images, cross-sectional height profile and load-displacement curve of the indent made on a Cu2O nanocube at a peak indentation load of 500 µN. Although the cube was heavily deformed, no crack debris or buckling was found. This indicates that the Cu2O nanocubes can be machined and manipulated for use as building blocks for micro/nanodevices. In summary, the hardness and elastic modulus of cuprous oxide nanocubes were measured by directly indenting the cubes. The hollow cube top wall acts as a membrane that bends under an indentation load. The Cu2O nanocubes are more ductile rather than brittle. It is also demonstrated that nanomachining of nanocubes is possible using a nanoindenter. Acknowledgment. Financial support for this study was provided by the National Science Foundation (Grant No. EPS-0296165), the University of South Carolina NanoCenter Seed Grant, and the South Carolina Space Grant ConsortiumNASA. The content of this information does not necessarily reflect the position or policy of the Government and no official endorsement should be inferred. References (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Snoke, D. Science 1996, 237, 1351. (3) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nature 2000, 407, 496. (4) Wang, W.; Wang, G.; Wang, X.; Zhan, Y.; Liu, Y.; Zheng, C. AdV. Mater. 2002, 14, 66. (5) Mishina, E. D.; Nagai, K.; Nakabayashi, S. Nano Lett. 2001, 1, 401404. (6) Gou, L.; Murphy, C. J. Nano Lett. 2003, 3, 231. (7) Wang, D.; Mo, M.; Yu, D.; Xu, L.; Li, F.; Qian, Y. Cryst. Growth Des. 2003, 3, 717. (8) Gou, L.; Murphy, J. C. J. Mater. Chem. 2004, 14, 735. (9) Li, X.; Bhushan, B. Surf. Coat. Technol. 2003, 163-164, 503. (10) Li, X.; Bhushan, B. Mater. Characterization 2002, 48, 11. (11) Bhushan, B.; Li, X. Int. Mater. ReV. 2003, 48, 125. (12) Li, X.; Gao, H.; Murphy, C. J.; Caswell, K. K. Nano Lett. 2003, 3, 1495. (13) Oliver, W. C.; Pharr, G. M. J. Mater. Res. 1992, 7, 1564. (14) Sneddon, I. N. Int. J. Eng. Sci. 1965, 3, 47-56. (15) 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.
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