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Mechanical characterization of aluminum doped zinc oxide (Al:ZnO) nanorods prepared by sol–gel method
Applied Surface Science 265 (2013) 758–763
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Mechanical characterization of aluminum doped zinc oxide (Al:ZnO) nanorods prepared by sol–gel method A. Kumar a,∗ , N. Huang b , T. Staedler a , C. Sun b , X. Jiang a a b
Institute of Materials Engineering, University of Siegen, Siegen 57076, Germany Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 28 October 2012 Accepted 18 November 2012 Available online 26 November 2012 Keywords: Nanorod Nanoindentation Mechanical properties Sol–gel
a b s t r a c t The surface and bucking instabilities of vertical well-aligned aluminum doped ZnO nanorods on the lime-glass substrates prepared using the sol–gel method are characterized by nanoindentation tests. Comprehensive structural analysis by X-ray diffraction and scanning electron microscopy reveal that the ZnO nanorods are grown as a single crystal along the [0 0 2] direction without any dislocation. Uniaxial compression tests of individual nanorods with the Berkovich and a conical indenter and of group of nanorods with flat punch indenter have been carried out. Using the Euler buckling model, the elastic moduli of ZnO nanorods using these three different indenters are within the range of 175–256 GPa. We discuss the relative merits of these two approaches for the determination of the elastic properties of ZnO nanorods, particularly considering the difference and difficulties of each approach. The ZnO nanorods prepared by the sol–gel method are mechanically strong and may assist the development of the applications of one dimensional nanorods. © 2012 Elsevier B.V. All rights reserved.
1. Introduction With advances in nanomaterials and nanotechnology, one dimensional semiconducting nanomaterials such as ZnO, NiO, Si–Ge, GaAs, and CdS, etc. particularly ZnO nanomaterials have attracted much attention due to their wide band gap, excellent chemical and thermal stabilities, chemical tailoring, and specific electrical and optical properties with large excitation binding energy. Considering this, ZnO nanomaterials can be used as building blocks for a broad range of high-technology applications ranging from light-emitting diodes, photodetectors, photocells, barristers, gas sensors, solar cells, and many other applications. The mechanical characterization of these nanostructures has attracted much attention due to its importance in nanoelectromechanical applications [1,2] as sometimes physical performance of materials cannot be optimized due to their poor mechanical stability. Therefore, it is important to investigate the mechanical characterization of ZnO film system to improve their reliability [1]. In general, the mechanical reliability includes strength, toughness, stiffness, hardness, and adhesion to the substrate. Among these characterizations, buckling and instability of vertical structures are very important for the consideration of design failure [3]. Many reports on the mechanical properties of ZnO nanorods have been published in the literature [2,4–12]. Young et al., Wen et al., and Riaz et al. [10–12] discussed
∗ Corresponding author. Tel.: +49 271 740 4065; fax: +49 271 740 2442. E-mail address: [email protected] (A. Kumar). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.101
the buckling characterization of ZnO nanorods by using the nanoindentation method. Young et al. and Wen et al. [10,11] have used the Euler model for the characterization of long vertical ZnO nanorods, while Riaz et al. [12] have used the Euler model as well as the J. B. Johnson model for the characterization of long and intermediate ZnO nanorods. However, there are also several reports on mechanical behavior of doped ZnO nanorods and nanostructures, which were grown using different techniques. For example, Xu et al. [13] reported buckling characterization of aligned Cr-doped ZnO nanorods fabricated using a vapor transport approach, Fang et al. [14] discussed the mechanical characterization of In-doped ZnO nanostructures synthesized via chemical solution method, and Fang et al. [15] reported physical characterization of Al-doped ZnO nanorods prepared by chemical solution method, etc. Despite of the above mentioned mechanical testing of vertical aligned ZnO nanorod and nanostructures prepared by different synthesizing techniques, aluminum doped ZnO nanorods synthesized via the sol–gel process were not evaluated so far [16,17], and hence their mechanical characterization was not done. Scanning nanoindentation technique has been primarily used as a technique to determine mechanical properties for nanotrusctues. With the ability to exert and measure forces up to the nanonewton load range, Nanoindentation is particularly suitable tool to determine the nanomechanical properties of nanometer-sized structures such as nanorods, nanowires, etc. In Nanoindentation, the tip approaches and indents the sample until a certain predefined force is reached. At this point the tip is retracted again. During this approach and retract cycle the force is continuously
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measured, resulting in a force versus distance graph. In general, the nanoindentation test would be on an individual nanostructure, which is, however, difficult to locate a single nanostructure. For such experiments, the tip will be very sharp. The accuracy of results obtained for nanoindentation test on an individual nanostructure is limited on nanostructure shape, size, density, and their distribution as well scanning capability of indenter system. This approach is typically performed with pre and post scan of surface, which makes this method inherently slow. Here, pre and post images are recorded, which results in data acquisition times of up to hour for a single indent. To circumvent this difficult experimental requirement, nanostructures can be tested by compressing a small group of nanostructures instead of a single nanostructure. Several studies on uniaxial compression testing of nanostructures using a flat punch nanoindentation system were published [18,19]. Here Nanoindentation is performed with a known geometry such as flat punch indenter. With little or no help of analysis, this approach can generate easily and faster innumerous data points. We used above both approaches, uniaxial compression test of individual and group of nanorods, to determine the modulus of elasticity of ZnO nanorods, those were synthesized using the sol–gel method with a proper aluminum doping level [16,17]. The Euler bucking model [20] is used to evaluate the mechanical properties of ZnO nanorods. In this work, we discussed the relative merits of the application of these two methods specifically for the determination of the elastic properties of ZnO nanorod in more detail, with particular emphasis on the differences and difficulties of each method.
2. Materials and experimental procedures The aluminum doped ZnO nanorods were grown by sol–gel method. The solution with 4 at.% aluminum content was prepared and the solution was stirred until it becomes the transparent and light-yellow. The detailed procedure of preparing sol solution has been described in detail in our previous work [16,17]. Later, the solution served as the starting solution after aging at room temperature for more than 24 h. Standard float glass (lime glass) substrates were used for the deposition. Prior to the deposition, the substrates were cleaned in an ultrasonic bath for 15 min with acetone, ethanol, and distilled water. The clean substrates were then dipped into the solution and withdrawn at a rate of 3.5 cm/min. The coated substrates were dried at 240 ◦ C for 20 min after each dipping. By repeating the procedure and changing the coating times, ZnO-based layers of different thicknesses were obtained. Finally, the coating layers were annealed in a tube furnace at 550 ◦ C for 60 min. X-ray diffraction (XRD) measurements of the ZnO nanorods film were carried out using a Cu K␣ radiation (wavelength: = 0.154 nm). Surface morphologies of the coating layers were observed by a JSM-6700F scanning electron microscope (SEM) tilted at 0◦ and 45◦ . The microstructure was characterized by a Tecnai G2 F30 transmission electron microscope (TEM). In addition, an X-ray photoelectron spectroscopy (XPS) was applied with the Al K␣ (h = 1286.6 eV) radiation using a RIBER LAS-3000, and the position of the C 1s peak was taken as the reference with a binding energy of 285 eV. The mechanical characterization of nanorods was performed by the means of a transducer based scanning nanoindenter (TriboIndenter, Hysitron Inc.) in a laboratory environment (RT and 40% RH). The uniaxial compression on the exposed nanorods was accomplished with a diamond Berkovich indenter of about 100 nm radius, a diamond conical indenter of about 600 nm radius (90◦ cone opening angle), and a flat punch with a radius of about 6 m. The appropriate topography of indenters was characterized by atomic force microscopy (AFM) (XE-100, PSIA). Non-contact AFM was used
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to obtain detailed information about surface roughness. The indenters were imaged with commercial tips featuring a nominal tip radius of 10 nm in a feedback-controlled mode on all three x-, y, and z-axes. A 5 m ×5 m with a pixel resolution of 512 × 512 was taken in order to drive the corresponding RMS roughness. The resulting roughness of the Berkovich and conical indenter was found to be negligible. The resulting roughness of flat punch indenter is about 34 nm, which is later used in calculation. The average axial load per nanorod was conducted to calculate the mechanical properties of the distorted ZnO nanorods during nanoindentation. The indentation test for a group of nanorods was done at 2000 N (loading and unloading rate were 200 N/s with holding time of 3 s) using the flat punch indenter. The indentation test on single nanorod was done at 200 N (loading and unloading rate were 20 N/s with a holding time of 3 s) using the Berkovich and a conical indenter. Prior to the indent, an individual nanorod was located by in-situ AFM scanning. The indentation tests for the nanorods were destructive in nature; therefore several tests at different areas of the sample were carried out to prove the repeatability.
3. Results The SEM images of ZnO nanorods show their height and size distribution as shown in Fig. 1(a) and (b). The diameter of nanorods varies from 50 nm to 150 nm with very few nanorods appearing to have larger and lower sizes. The length of nanorods varies from 400 nm to 600 nm and few much shorter as well as longer nanorods are also seen. The average value of the nanorod diameter and length has been used in the calculation. The ZnO nanorods are well uniformly distributed over the substrate surface. The density of ZnO nanorods on the substrate is about 19 nanorods/m2 and the average distance between two nearest nanorods is about 253 nm. The SEM images also reveal that tops of these nanorods are hexagonal with the c-axis perpendicular to the substrate surface. From the SEM image it can be seen that only few nanorods are bending and not perfectly straight. The XRD diffraction patterns confirm that the ZnO nanorod is a single crystal and grows along the [0 0 2] c-axis direction without any dislocation as shown in Fig. 1(c). The compression tests on the exposed ZnO nanorods have been accomplished using nanoindenters. The ZnO nanorods are loaded to a prescribed force in a force-controlled mode. The typical loaddisplacement curve for ZnO nanorods is shown in Fig. 2. In general, it has three segments: loading, holding, and unloading. It is a well-known phenomenon of ideal vertically inclined columns or nanorods compressed by an axial load; the loading portion consists of three stages: an initial increase in slope, sudden drop in slope and curve becoming flat, and further increasing the slope. In other terms, (1) if load (P) is less than critical load (Pcr ) i.e. P < Pcr , the column is stable in the straight position; (2) if P = Pcr , the column is in neutral equilibrium in either the straight or slightly bent position; and (3) if P > Pcr , the column is unstable in the straight position and will buckle under the slightest disturbance, as shown in Fig. 2. This type of buckling is a perfect example of the Euler buckling [20]. In the Euler buckling model, the critical load is calculated by Pcr = 2 EI/Le2 , where E is the Young’s modulus and I is the moment of inertia (=R4 /4) for the vertical nanorods. R is the radius of nanorods in the present work and the effective length Le is expressed in terms of an effective length factor K as Le = KL, where L is the actual length of the nanorods. The buckling phenomena can be occurred with two possible conditions: (1) K = 0.5, considering both ends of nanorod columns are fixed against rotation i.e. fixed-fixed column; (2) K = 0.7, considering one end of the nanorod is fixed and the other top end is pinned i.e. fixed-pinned column. In the presented work, we have used 0.7 as K in the calculation.
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The critical buckling strain is given by εcr = cr /E, where the critical buckling stress cr = Pcr /A. Applying the Euler buckling model, some distinct differences in the Young’s modulus values obtained with from three kinds of indenters, which is a subject of study in this work, are revealed.
3.1. Uniaxial compression of individual ZnO nanorod The uniaxial compression tests on a single ZnO nanorod have been performed using the Berkovich and conical indenter. Although the interpretation of the results for conical tip becomes complicated because all nanorods under compression are typically not loaded to the same extent, but it is possible to extract the critical load for a single nanorod with known geometry of the conical indenter. Since the density of nanorods on the substrate surface in present work is not very high, the conical indenter can be loaded on one nanorod at the applied load of 200 N, and this is confirmed via post scanning image; see in Fig. 4 (b). Thus, the average critical load for a single nanorod is directly measured from the force-displacement curve as shown in Fig. 4(c). Image resolution obtained by scanning with Berkovich and conical indenter is not very high; however, it is enough to locate the nanorods unambiguously. Pre and post scan have been done to locate the nanorods as shown in Figs. 3 and 4(a) and (b). After imaging, the indenter is instructed to perform the test for a single column in load control mode. The nanorods are loaded to a prescribed force and then unloaded. The same scan area is again scanned to confirm that the test is done on the targeted nanorod, and not between the two rods as shown in Fig. 3(a) and (b) and Fig. 4(a) and (b). The force-displacement curve for ZnO nanorods using the Berkovich and a conical indenter are shown in Fig. 3(c) and Fig. 4(c). Average critical load of 38 N is found when a single ZnO nanorod is compressed. The buckling behavior of the nanorods is derived by considering a nanocolumn fixed at the base and pinned at the top, and K = 0.7. Average Young’s modulus (E) of ZnO nanorods is found to be 175 GPa. Further, the critical stress and strain are also calculated as shown in Table 1.
3.2. Uniaxial compression of group of ZnO nanorods
Fig. 1. (a) SEM morphology, (b) cross-sectional SEM image, and (c) corresponding XRD spectra of ZnO nanorods.
Fig. 2. Load vs. displacement curve of aluminum doped ZnO nanorods from nanoindentation experiments using flat punch indenter. The curve shows buckling instability of ZnO nanorods. (1) P < Pcr , the column is stable; (2) if P = Pcr , the column is in neutral equilibrium; and (3) if P > Pcr , the column is unstable.
In order to compress a group of nanorods, a flat punch indenter of 6 m radius has been used. The flat punch indenter shows the problem of parallel alignment of flat punch indenter to the substrate plane and it results in uncertainty of number of nanorods beneath the indenter. This issue can be solved by proper adjusting the flat punch before the experiments. In compression of group of nanorods with flat punch indenter, pre and post scanning of surface does not require. The compression test was performed by making 5 × 8 grid with 30 m spacing which resulting 40 forcedisplacement curves. In analyzing the force-displacement curve, instead of single drop in the loading curve, the multiple buckling stages (drops in curve) at different loads are observed as shown in Fig. 5(b). As mentioned earlier, the height of the nanorods is not uniform; therefore all nanorods have not been compressed at the same time. On the loading indenter, the first compression is done for the highest nanorod and afterward the indenter compresses the shorter nanorods. Finally it compresses the shortest nanorods. The actual number of nanorods is varied according to the indenter depth and the actual number of nanorods at each buckling level is statistically counted using Fig. 5(a). We have adjusted the indenter depth by considering the critical indenter depth of individual nanorod before starting to fracture and the average roughness of flat punch surface. The average critical loads are observed at 40, 243, 533, and 1250 N. The numbers of nanorods are statistically counted at each stage of buckling and by applying the Euler buckling
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Fig. 3. (a) 2 m × 2 m AFM image before indent and (b) AFM image after indent. Circles in (a) and (b) show the nanorod position before and after indent, respectively. (c) Load vs. displacement curve aluminum doped ZnO nanorods from nanoindentation experiments using Berkovich indenter.
model, the Young’s modulus (E) of the ZnO nanorods is 256, 218, 205, and 106 GPa at different level of buckling (Table 1). 4. Discussion In this work the mechanical properties of aluminum doped ZnO nanorods with Berkovich, conical, and flat punch indenter have been characterized. The Euler buckling model is employed to characterize the mechanical properties of the nanorods. The critical stress, critical buckling strain, and the Young’s modulus of the
ZnO nanorods using three indenters are presented in Table 1. The Berkovich and conical indenter are generated an average Young’s modulus of 175 GPa. Analyzing the force–displacement produced by the flat punch indenter and height variation of nanorods on the surface as shown in Fig. 5, four different Young’s modulus values at different indenter depths are obtained as 256, 218, 205, and 106 GPa. The fourth buckling is not properly established as shown in Fig. 5(b). It may possible that flat punch may already crush most of the nanorods at 1250 N load and it may generate the Young’s modulus of bulk ZnO rather than ZnO nanorods. Other three Young’s
Table 1 Critical load, stress, buckling strain, and Young’s modulus of ZnO nanorods using the Berkovich, a conical, and a flat punch indenter. Tip shape/radius (nm)
Average critical load, Pcr (N) 38.56 ± 5.61 38.29 ± 3.86
Berkovich/100 Conical/600 Flat punch/6000
First buckling Second buckling Third buckling Fourth buckling
40.33 243.00 532.50 1250.50
± ± ± ±
20.42 68.46 105.93 220.28
Yield strength, cr (GPa)
Strain, εcr (%)
Young’s modulus, E (GPa)
7.30 ± 1.06 7.25 ± 0.73
4.12 ± 0.60 4.13 ± 0.42
177.00 ± 25.78 175.76 ± 17.72
7.64 6.57 6.72 3.88
± ± ± ±
3.87 1.85 1.34 0.68
2.98 3.02 3.28 3.65
± ± ± ±
1.51 0.85 0.65 0.64
256.30 217.80 204.91 106.18
± ± ± ±
129.77 61.36 40.76 18.71
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Fig. 4. (a) 10 m × 10 m AFM image before indent and (b) AFM image after indent. Circles in (a) and (b) show the nanorod position before and after indent, respectively. (c) Load vs. displacement curve aluminum doped ZnO nanorods from nanoindentation experiments using conical indenter.
moduli are little higher than the value obtained from the Berkovich and conical indenter. This is mainly due to different measurement approaches. Both approaches (individual and group of nanorod compression tests) have own advantages and difficulties in measurements. In individual nanorod compression test by Berkovich and conical indenter, the surface is pre and post scanned such that compression tests are performed only on single nanorod. This approach can provide accurate elastic modulus if length and size of nanorods are uniform. Image resolution obtained by scanning with Berkovich and conical indenters is not very high and the exact size of compressed nanorod is extremely difficult to measure. In present study the nanorod length varies from 400 to 600 nm and diameter varies from 50 to 150 nm. Using their average values of length and diameter in calculating mechanical properties will lead the wrong values. This approach is a time consuming process because it requires pre and post scanning in locating an individual nanostructure and this is very much trivial in case of very small size of structure for example nanotubes and nanowires. The Berkovich and conical indenter can also give wrong values because they can probe on the edge of nanorods rather than center of top of nanorods. In compression of a group of nanorod by flat punch indenter approach, it does not require any surface scanning. Flat
punch indenters can easily and fast innumerous data points, which can later produce valuable information. If height and distribution of nanostructures on substrate are homogeneous, then in no time, this approach can give accurate mechanical properties. This approach can also mislead the information if size and distribution of nanostructure are not uniform. This is observed in present study multiple buckling stages instead of single buckling. However with little use of analysis it can also generate accurate results. In both approaches, uniaxial compression of group of nanorods technique with flat punch is more effective and more certain to generate the specific mechanical properties of nanostructures. Comparing our results with previous study [21], the Young’s modulus of the pure ZnO films was about 64–128 GPa. The Young’s modulus of the undoped ZnO nanorods were measured as 21.6 GPa [13] and 12.1 GPa [12] using fixed pinned end condition and the Young’s modulus in the present work is about 10–20 times higher which may be due to the difference in geometrical features (length, thickness, etc.). For typical bulk ZnO, the Young’s modulus is about 144 GPa, however, the Young’s modulus in the present work is higher than the bulk values of the ZnO (144 GPa), which could be explained by Hall-Petch effect [22,23], in that the smaller grain size of materials had higher material strength.
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grows along the [0 0 2] direction without any dislocation. The ZnO nanorods are investigated by nanoindentation system and uniaxial compression tests were performed on an individual (with Berkovich and conical indenter) and a group (with flat punch indenter) of ZnO nanorods. The flat punch indenter is produced multiple buckling stages depending on the indenter depth and produced different values of the Young’s modulus. The Euler buckling model is employed for evaluating the Young’s modulus of the individual ZnO nanorod. The single ZnO nanorod compression test is more accurate but time consuming. The group of ZnO nanorods compression test is easy and fast but it requires analysis. The present work shows that the ZnO nanorods prepared by the sol–gel method are mechanically strong and may assist the development of the applications of one dimensional nanorods. References
Fig. 5. (a) Cumulative number of nanorods vs. height of nanorod, (b) load vs. displacement curve aluminum doped ZnO nanorods from nanoindentation experiments using flat punch indenter.
In order to compare our results with the doped ZnO nanorods or nanostructures, Xu et al. [13] reported the buckling characterization of aligned Cr-doped ZnO nanorods fabricated using a vapor transport approach and found 249.7 GPa Young’s modulus, Fang et al. [14] discussed the mechanical characterization of different levels of the In-doped ZnO nanostructures synthesized via the chemical solution method and their Young’s modulus were in between 64.6 and 345.7 GPa. Fang et al. [15] also reported the physical characterization of Al-doped ZnO nanorods prepared by the chemical solution method and found that the Young’s modulus varied as 40.38–628.93 GPa with changing the Al doping level, etc. In the present work, the Young’s modulus of the ZnO based nanorods on glass substrates prepared with the sol–gel method with proper aluminum doping, is comparable with above previous studies of different doped ZnO nanostructure. The highlighted feature within the presented work is the preparation technique, which opens the door for a cost effective preparation method of ZnO nanorods based films with stronger mechanical strength for many applications. 5. Summary The mechanical characterization of vertical well-aligned single crystal aluminum doped ZnO nanorods on lime-glass substrates prepared by the sol–gel method is reported. Comprehensive structural analysis showed that ZnO nanorod is a single crystal and
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Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Mechanical characterization of aluminum doped zinc oxide (Al:ZnO) nanorods prepared by sol–gel method A. Kumar a,∗ , N. Huang b , T. Staedler a , C. Sun b , X. Jiang a a b
Institute of Materials Engineering, University of Siegen, Siegen 57076, Germany Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 28 October 2012 Accepted 18 November 2012 Available online 26 November 2012 Keywords: Nanorod Nanoindentation Mechanical properties Sol–gel
a b s t r a c t The surface and bucking instabilities of vertical well-aligned aluminum doped ZnO nanorods on the lime-glass substrates prepared using the sol–gel method are characterized by nanoindentation tests. Comprehensive structural analysis by X-ray diffraction and scanning electron microscopy reveal that the ZnO nanorods are grown as a single crystal along the [0 0 2] direction without any dislocation. Uniaxial compression tests of individual nanorods with the Berkovich and a conical indenter and of group of nanorods with flat punch indenter have been carried out. Using the Euler buckling model, the elastic moduli of ZnO nanorods using these three different indenters are within the range of 175–256 GPa. We discuss the relative merits of these two approaches for the determination of the elastic properties of ZnO nanorods, particularly considering the difference and difficulties of each approach. The ZnO nanorods prepared by the sol–gel method are mechanically strong and may assist the development of the applications of one dimensional nanorods. © 2012 Elsevier B.V. All rights reserved.
1. Introduction With advances in nanomaterials and nanotechnology, one dimensional semiconducting nanomaterials such as ZnO, NiO, Si–Ge, GaAs, and CdS, etc. particularly ZnO nanomaterials have attracted much attention due to their wide band gap, excellent chemical and thermal stabilities, chemical tailoring, and specific electrical and optical properties with large excitation binding energy. Considering this, ZnO nanomaterials can be used as building blocks for a broad range of high-technology applications ranging from light-emitting diodes, photodetectors, photocells, barristers, gas sensors, solar cells, and many other applications. The mechanical characterization of these nanostructures has attracted much attention due to its importance in nanoelectromechanical applications [1,2] as sometimes physical performance of materials cannot be optimized due to their poor mechanical stability. Therefore, it is important to investigate the mechanical characterization of ZnO film system to improve their reliability [1]. In general, the mechanical reliability includes strength, toughness, stiffness, hardness, and adhesion to the substrate. Among these characterizations, buckling and instability of vertical structures are very important for the consideration of design failure [3]. Many reports on the mechanical properties of ZnO nanorods have been published in the literature [2,4–12]. Young et al., Wen et al., and Riaz et al. [10–12] discussed
∗ Corresponding author. Tel.: +49 271 740 4065; fax: +49 271 740 2442. E-mail address: [email protected] (A. Kumar). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.101
the buckling characterization of ZnO nanorods by using the nanoindentation method. Young et al. and Wen et al. [10,11] have used the Euler model for the characterization of long vertical ZnO nanorods, while Riaz et al. [12] have used the Euler model as well as the J. B. Johnson model for the characterization of long and intermediate ZnO nanorods. However, there are also several reports on mechanical behavior of doped ZnO nanorods and nanostructures, which were grown using different techniques. For example, Xu et al. [13] reported buckling characterization of aligned Cr-doped ZnO nanorods fabricated using a vapor transport approach, Fang et al. [14] discussed the mechanical characterization of In-doped ZnO nanostructures synthesized via chemical solution method, and Fang et al. [15] reported physical characterization of Al-doped ZnO nanorods prepared by chemical solution method, etc. Despite of the above mentioned mechanical testing of vertical aligned ZnO nanorod and nanostructures prepared by different synthesizing techniques, aluminum doped ZnO nanorods synthesized via the sol–gel process were not evaluated so far [16,17], and hence their mechanical characterization was not done. Scanning nanoindentation technique has been primarily used as a technique to determine mechanical properties for nanotrusctues. With the ability to exert and measure forces up to the nanonewton load range, Nanoindentation is particularly suitable tool to determine the nanomechanical properties of nanometer-sized structures such as nanorods, nanowires, etc. In Nanoindentation, the tip approaches and indents the sample until a certain predefined force is reached. At this point the tip is retracted again. During this approach and retract cycle the force is continuously
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measured, resulting in a force versus distance graph. In general, the nanoindentation test would be on an individual nanostructure, which is, however, difficult to locate a single nanostructure. For such experiments, the tip will be very sharp. The accuracy of results obtained for nanoindentation test on an individual nanostructure is limited on nanostructure shape, size, density, and their distribution as well scanning capability of indenter system. This approach is typically performed with pre and post scan of surface, which makes this method inherently slow. Here, pre and post images are recorded, which results in data acquisition times of up to hour for a single indent. To circumvent this difficult experimental requirement, nanostructures can be tested by compressing a small group of nanostructures instead of a single nanostructure. Several studies on uniaxial compression testing of nanostructures using a flat punch nanoindentation system were published [18,19]. Here Nanoindentation is performed with a known geometry such as flat punch indenter. With little or no help of analysis, this approach can generate easily and faster innumerous data points. We used above both approaches, uniaxial compression test of individual and group of nanorods, to determine the modulus of elasticity of ZnO nanorods, those were synthesized using the sol–gel method with a proper aluminum doping level [16,17]. The Euler bucking model [20] is used to evaluate the mechanical properties of ZnO nanorods. In this work, we discussed the relative merits of the application of these two methods specifically for the determination of the elastic properties of ZnO nanorod in more detail, with particular emphasis on the differences and difficulties of each method.
2. Materials and experimental procedures The aluminum doped ZnO nanorods were grown by sol–gel method. The solution with 4 at.% aluminum content was prepared and the solution was stirred until it becomes the transparent and light-yellow. The detailed procedure of preparing sol solution has been described in detail in our previous work [16,17]. Later, the solution served as the starting solution after aging at room temperature for more than 24 h. Standard float glass (lime glass) substrates were used for the deposition. Prior to the deposition, the substrates were cleaned in an ultrasonic bath for 15 min with acetone, ethanol, and distilled water. The clean substrates were then dipped into the solution and withdrawn at a rate of 3.5 cm/min. The coated substrates were dried at 240 ◦ C for 20 min after each dipping. By repeating the procedure and changing the coating times, ZnO-based layers of different thicknesses were obtained. Finally, the coating layers were annealed in a tube furnace at 550 ◦ C for 60 min. X-ray diffraction (XRD) measurements of the ZnO nanorods film were carried out using a Cu K␣ radiation (wavelength: = 0.154 nm). Surface morphologies of the coating layers were observed by a JSM-6700F scanning electron microscope (SEM) tilted at 0◦ and 45◦ . The microstructure was characterized by a Tecnai G2 F30 transmission electron microscope (TEM). In addition, an X-ray photoelectron spectroscopy (XPS) was applied with the Al K␣ (h = 1286.6 eV) radiation using a RIBER LAS-3000, and the position of the C 1s peak was taken as the reference with a binding energy of 285 eV. The mechanical characterization of nanorods was performed by the means of a transducer based scanning nanoindenter (TriboIndenter, Hysitron Inc.) in a laboratory environment (RT and 40% RH). The uniaxial compression on the exposed nanorods was accomplished with a diamond Berkovich indenter of about 100 nm radius, a diamond conical indenter of about 600 nm radius (90◦ cone opening angle), and a flat punch with a radius of about 6 m. The appropriate topography of indenters was characterized by atomic force microscopy (AFM) (XE-100, PSIA). Non-contact AFM was used
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to obtain detailed information about surface roughness. The indenters were imaged with commercial tips featuring a nominal tip radius of 10 nm in a feedback-controlled mode on all three x-, y, and z-axes. A 5 m ×5 m with a pixel resolution of 512 × 512 was taken in order to drive the corresponding RMS roughness. The resulting roughness of the Berkovich and conical indenter was found to be negligible. The resulting roughness of flat punch indenter is about 34 nm, which is later used in calculation. The average axial load per nanorod was conducted to calculate the mechanical properties of the distorted ZnO nanorods during nanoindentation. The indentation test for a group of nanorods was done at 2000 N (loading and unloading rate were 200 N/s with holding time of 3 s) using the flat punch indenter. The indentation test on single nanorod was done at 200 N (loading and unloading rate were 20 N/s with a holding time of 3 s) using the Berkovich and a conical indenter. Prior to the indent, an individual nanorod was located by in-situ AFM scanning. The indentation tests for the nanorods were destructive in nature; therefore several tests at different areas of the sample were carried out to prove the repeatability.
3. Results The SEM images of ZnO nanorods show their height and size distribution as shown in Fig. 1(a) and (b). The diameter of nanorods varies from 50 nm to 150 nm with very few nanorods appearing to have larger and lower sizes. The length of nanorods varies from 400 nm to 600 nm and few much shorter as well as longer nanorods are also seen. The average value of the nanorod diameter and length has been used in the calculation. The ZnO nanorods are well uniformly distributed over the substrate surface. The density of ZnO nanorods on the substrate is about 19 nanorods/m2 and the average distance between two nearest nanorods is about 253 nm. The SEM images also reveal that tops of these nanorods are hexagonal with the c-axis perpendicular to the substrate surface. From the SEM image it can be seen that only few nanorods are bending and not perfectly straight. The XRD diffraction patterns confirm that the ZnO nanorod is a single crystal and grows along the [0 0 2] c-axis direction without any dislocation as shown in Fig. 1(c). The compression tests on the exposed ZnO nanorods have been accomplished using nanoindenters. The ZnO nanorods are loaded to a prescribed force in a force-controlled mode. The typical loaddisplacement curve for ZnO nanorods is shown in Fig. 2. In general, it has three segments: loading, holding, and unloading. It is a well-known phenomenon of ideal vertically inclined columns or nanorods compressed by an axial load; the loading portion consists of three stages: an initial increase in slope, sudden drop in slope and curve becoming flat, and further increasing the slope. In other terms, (1) if load (P) is less than critical load (Pcr ) i.e. P < Pcr , the column is stable in the straight position; (2) if P = Pcr , the column is in neutral equilibrium in either the straight or slightly bent position; and (3) if P > Pcr , the column is unstable in the straight position and will buckle under the slightest disturbance, as shown in Fig. 2. This type of buckling is a perfect example of the Euler buckling [20]. In the Euler buckling model, the critical load is calculated by Pcr = 2 EI/Le2 , where E is the Young’s modulus and I is the moment of inertia (=R4 /4) for the vertical nanorods. R is the radius of nanorods in the present work and the effective length Le is expressed in terms of an effective length factor K as Le = KL, where L is the actual length of the nanorods. The buckling phenomena can be occurred with two possible conditions: (1) K = 0.5, considering both ends of nanorod columns are fixed against rotation i.e. fixed-fixed column; (2) K = 0.7, considering one end of the nanorod is fixed and the other top end is pinned i.e. fixed-pinned column. In the presented work, we have used 0.7 as K in the calculation.
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The critical buckling strain is given by εcr = cr /E, where the critical buckling stress cr = Pcr /A. Applying the Euler buckling model, some distinct differences in the Young’s modulus values obtained with from three kinds of indenters, which is a subject of study in this work, are revealed.
3.1. Uniaxial compression of individual ZnO nanorod The uniaxial compression tests on a single ZnO nanorod have been performed using the Berkovich and conical indenter. Although the interpretation of the results for conical tip becomes complicated because all nanorods under compression are typically not loaded to the same extent, but it is possible to extract the critical load for a single nanorod with known geometry of the conical indenter. Since the density of nanorods on the substrate surface in present work is not very high, the conical indenter can be loaded on one nanorod at the applied load of 200 N, and this is confirmed via post scanning image; see in Fig. 4 (b). Thus, the average critical load for a single nanorod is directly measured from the force-displacement curve as shown in Fig. 4(c). Image resolution obtained by scanning with Berkovich and conical indenter is not very high; however, it is enough to locate the nanorods unambiguously. Pre and post scan have been done to locate the nanorods as shown in Figs. 3 and 4(a) and (b). After imaging, the indenter is instructed to perform the test for a single column in load control mode. The nanorods are loaded to a prescribed force and then unloaded. The same scan area is again scanned to confirm that the test is done on the targeted nanorod, and not between the two rods as shown in Fig. 3(a) and (b) and Fig. 4(a) and (b). The force-displacement curve for ZnO nanorods using the Berkovich and a conical indenter are shown in Fig. 3(c) and Fig. 4(c). Average critical load of 38 N is found when a single ZnO nanorod is compressed. The buckling behavior of the nanorods is derived by considering a nanocolumn fixed at the base and pinned at the top, and K = 0.7. Average Young’s modulus (E) of ZnO nanorods is found to be 175 GPa. Further, the critical stress and strain are also calculated as shown in Table 1.
3.2. Uniaxial compression of group of ZnO nanorods
Fig. 1. (a) SEM morphology, (b) cross-sectional SEM image, and (c) corresponding XRD spectra of ZnO nanorods.
Fig. 2. Load vs. displacement curve of aluminum doped ZnO nanorods from nanoindentation experiments using flat punch indenter. The curve shows buckling instability of ZnO nanorods. (1) P < Pcr , the column is stable; (2) if P = Pcr , the column is in neutral equilibrium; and (3) if P > Pcr , the column is unstable.
In order to compress a group of nanorods, a flat punch indenter of 6 m radius has been used. The flat punch indenter shows the problem of parallel alignment of flat punch indenter to the substrate plane and it results in uncertainty of number of nanorods beneath the indenter. This issue can be solved by proper adjusting the flat punch before the experiments. In compression of group of nanorods with flat punch indenter, pre and post scanning of surface does not require. The compression test was performed by making 5 × 8 grid with 30 m spacing which resulting 40 forcedisplacement curves. In analyzing the force-displacement curve, instead of single drop in the loading curve, the multiple buckling stages (drops in curve) at different loads are observed as shown in Fig. 5(b). As mentioned earlier, the height of the nanorods is not uniform; therefore all nanorods have not been compressed at the same time. On the loading indenter, the first compression is done for the highest nanorod and afterward the indenter compresses the shorter nanorods. Finally it compresses the shortest nanorods. The actual number of nanorods is varied according to the indenter depth and the actual number of nanorods at each buckling level is statistically counted using Fig. 5(a). We have adjusted the indenter depth by considering the critical indenter depth of individual nanorod before starting to fracture and the average roughness of flat punch surface. The average critical loads are observed at 40, 243, 533, and 1250 N. The numbers of nanorods are statistically counted at each stage of buckling and by applying the Euler buckling
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Fig. 3. (a) 2 m × 2 m AFM image before indent and (b) AFM image after indent. Circles in (a) and (b) show the nanorod position before and after indent, respectively. (c) Load vs. displacement curve aluminum doped ZnO nanorods from nanoindentation experiments using Berkovich indenter.
model, the Young’s modulus (E) of the ZnO nanorods is 256, 218, 205, and 106 GPa at different level of buckling (Table 1). 4. Discussion In this work the mechanical properties of aluminum doped ZnO nanorods with Berkovich, conical, and flat punch indenter have been characterized. The Euler buckling model is employed to characterize the mechanical properties of the nanorods. The critical stress, critical buckling strain, and the Young’s modulus of the
ZnO nanorods using three indenters are presented in Table 1. The Berkovich and conical indenter are generated an average Young’s modulus of 175 GPa. Analyzing the force–displacement produced by the flat punch indenter and height variation of nanorods on the surface as shown in Fig. 5, four different Young’s modulus values at different indenter depths are obtained as 256, 218, 205, and 106 GPa. The fourth buckling is not properly established as shown in Fig. 5(b). It may possible that flat punch may already crush most of the nanorods at 1250 N load and it may generate the Young’s modulus of bulk ZnO rather than ZnO nanorods. Other three Young’s
Table 1 Critical load, stress, buckling strain, and Young’s modulus of ZnO nanorods using the Berkovich, a conical, and a flat punch indenter. Tip shape/radius (nm)
Average critical load, Pcr (N) 38.56 ± 5.61 38.29 ± 3.86
Berkovich/100 Conical/600 Flat punch/6000
First buckling Second buckling Third buckling Fourth buckling
40.33 243.00 532.50 1250.50
± ± ± ±
20.42 68.46 105.93 220.28
Yield strength, cr (GPa)
Strain, εcr (%)
Young’s modulus, E (GPa)
7.30 ± 1.06 7.25 ± 0.73
4.12 ± 0.60 4.13 ± 0.42
177.00 ± 25.78 175.76 ± 17.72
7.64 6.57 6.72 3.88
± ± ± ±
3.87 1.85 1.34 0.68
2.98 3.02 3.28 3.65
± ± ± ±
1.51 0.85 0.65 0.64
256.30 217.80 204.91 106.18
± ± ± ±
129.77 61.36 40.76 18.71
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Fig. 4. (a) 10 m × 10 m AFM image before indent and (b) AFM image after indent. Circles in (a) and (b) show the nanorod position before and after indent, respectively. (c) Load vs. displacement curve aluminum doped ZnO nanorods from nanoindentation experiments using conical indenter.
moduli are little higher than the value obtained from the Berkovich and conical indenter. This is mainly due to different measurement approaches. Both approaches (individual and group of nanorod compression tests) have own advantages and difficulties in measurements. In individual nanorod compression test by Berkovich and conical indenter, the surface is pre and post scanned such that compression tests are performed only on single nanorod. This approach can provide accurate elastic modulus if length and size of nanorods are uniform. Image resolution obtained by scanning with Berkovich and conical indenters is not very high and the exact size of compressed nanorod is extremely difficult to measure. In present study the nanorod length varies from 400 to 600 nm and diameter varies from 50 to 150 nm. Using their average values of length and diameter in calculating mechanical properties will lead the wrong values. This approach is a time consuming process because it requires pre and post scanning in locating an individual nanostructure and this is very much trivial in case of very small size of structure for example nanotubes and nanowires. The Berkovich and conical indenter can also give wrong values because they can probe on the edge of nanorods rather than center of top of nanorods. In compression of a group of nanorod by flat punch indenter approach, it does not require any surface scanning. Flat
punch indenters can easily and fast innumerous data points, which can later produce valuable information. If height and distribution of nanostructures on substrate are homogeneous, then in no time, this approach can give accurate mechanical properties. This approach can also mislead the information if size and distribution of nanostructure are not uniform. This is observed in present study multiple buckling stages instead of single buckling. However with little use of analysis it can also generate accurate results. In both approaches, uniaxial compression of group of nanorods technique with flat punch is more effective and more certain to generate the specific mechanical properties of nanostructures. Comparing our results with previous study [21], the Young’s modulus of the pure ZnO films was about 64–128 GPa. The Young’s modulus of the undoped ZnO nanorods were measured as 21.6 GPa [13] and 12.1 GPa [12] using fixed pinned end condition and the Young’s modulus in the present work is about 10–20 times higher which may be due to the difference in geometrical features (length, thickness, etc.). For typical bulk ZnO, the Young’s modulus is about 144 GPa, however, the Young’s modulus in the present work is higher than the bulk values of the ZnO (144 GPa), which could be explained by Hall-Petch effect [22,23], in that the smaller grain size of materials had higher material strength.
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grows along the [0 0 2] direction without any dislocation. The ZnO nanorods are investigated by nanoindentation system and uniaxial compression tests were performed on an individual (with Berkovich and conical indenter) and a group (with flat punch indenter) of ZnO nanorods. The flat punch indenter is produced multiple buckling stages depending on the indenter depth and produced different values of the Young’s modulus. The Euler buckling model is employed for evaluating the Young’s modulus of the individual ZnO nanorod. The single ZnO nanorod compression test is more accurate but time consuming. The group of ZnO nanorods compression test is easy and fast but it requires analysis. The present work shows that the ZnO nanorods prepared by the sol–gel method are mechanically strong and may assist the development of the applications of one dimensional nanorods. References
Fig. 5. (a) Cumulative number of nanorods vs. height of nanorod, (b) load vs. displacement curve aluminum doped ZnO nanorods from nanoindentation experiments using flat punch indenter.
In order to compare our results with the doped ZnO nanorods or nanostructures, Xu et al. [13] reported the buckling characterization of aligned Cr-doped ZnO nanorods fabricated using a vapor transport approach and found 249.7 GPa Young’s modulus, Fang et al. [14] discussed the mechanical characterization of different levels of the In-doped ZnO nanostructures synthesized via the chemical solution method and their Young’s modulus were in between 64.6 and 345.7 GPa. Fang et al. [15] also reported the physical characterization of Al-doped ZnO nanorods prepared by the chemical solution method and found that the Young’s modulus varied as 40.38–628.93 GPa with changing the Al doping level, etc. In the present work, the Young’s modulus of the ZnO based nanorods on glass substrates prepared with the sol–gel method with proper aluminum doping, is comparable with above previous studies of different doped ZnO nanostructure. The highlighted feature within the presented work is the preparation technique, which opens the door for a cost effective preparation method of ZnO nanorods based films with stronger mechanical strength for many applications. 5. Summary The mechanical characterization of vertical well-aligned single crystal aluminum doped ZnO nanorods on lime-glass substrates prepared by the sol–gel method is reported. Comprehensive structural analysis showed that ZnO nanorod is a single crystal and
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