Nanomechanical characterization of one-dimensional nanostructures

Abstract

One-dimensional (1D) nanostructures belong to an important class of nanomaterials. Since the discovery of carbon nanotubes by Iijima in 1991 [1], 1D nanostructures such as nanotubes [2], nanowires [3], nanorods [4,5], nanobelts [6], nanoribbons [7], and nanofibers [8] have been synthesized and widely investigated. Studies have shown that these nanostructures possess not only unique electrical, thermal, and optical properties, but also outstanding mechanical properties [9, 10]. Nanomechanical characterization of 1D nanostructures is so important that it will determine their applications in nanotechnology such as nanoresonators [11], biological sensors [12], nanocantilever [13], piezoelectric nanogenerators [14], nanoelectromechanical systems (NEMS), and tissue engineered scaffolds [15, 16]. It is of great importance to measure the mechanical properties directly from these nanostructures since the properties might have size-dependent behavior at the nanometer length scale. This implies that the mechanical properties of nanostructures cannot be accurately extrapolated from that of the bulk materials. In this chapter, experimental techniques used in the nanomechanical characterization of 1D nanostructures will be presented. Approaches used to measure the mechanical properties of 1D nanostructures will be the main focus of this chapter. Sample preparation and the challenges in conducting such tests are also discussed. Experimental study on the mechanical properties of 1D nanostructures is extremely challenging due to their lateral dimensions being a few tens of nanometers and longitudinal dimensions of a few microns. Taking the elastic modulus of ZnO nanowires as an example, different methods yield different results as shown in Table 5.1. The main challenges for quantitative measurement of the mechanical properties include (i) designing an appropriate test configuration such as fabrication of substrate, scattering/picking/placing/ clamping of specimens; (ii) applying and measurement of forces in the nano-Newton level; and (iii) measuring the mechanical deformation at the nanometer length scale. By utilizing the advantages of various types of high-resolution microscopes, nanoscale testing techniques have been developed. These high-resolution microscopes include scanning electron microscope (SEM), transmission electron microscope (TEM), and atomic force microscope (AFM) [21]. In order to manipulate and position small 1D nanostructures under electron microscopes, nanomanipulator with probes [22, 23] or MEMS-based nanoscale material testing system (n-MTS) [24] must be installed in these microscopes. The probes can be AFM tips or tungsten tips with radius of tens to a few hundred nanometers. AFM tips serve as force sensors and tungsten tips are used as picking/placing tools, counterpart electrode, or sample substrate. The microscope will provide the function for measuring the mechanical deformation and specimen dimensions at the nanometer length scale. The n-MTS incorporates a capacitive sensor to measure load electronically at a high resolution. As for the AFM, nano-Newton force can be determined from the deflection of the AFM cantilever multiplied by the cantilever spring constant. The AFM cantilever spring constants range from the order of 0.01 to 100N m-1, which can be accurately calibrated. Highresolution mapping of surface morphology on almost any type of conductive or nonconductive material can be achieved with AFM. This also provides the information of sample dimensions. Methods developed and used for measuring the mechanical properties of individual 1D nanostructures can be classified into two different types based on the fundamental instruments previously mentioned: AFM-based and in situ electronmicroscopy-based testing [25]. Three-point bend test, lateral force microscopy (LFM), and nanoindentation test are classified under the first AFM-based technique; and mechanical resonance, static axial tensile stretching, and compression/buckling tests are classified under the second. For generality, mechanical tests on nanowires are used here for the demonstration of Mechanical Character gates of 1D nanostructures. © Springer Science+Business Media, LLC, 2008.