Nanoscale testing of one-dimensional nanostructures

Abstract

The emergence of numerous nanoscale materials and structures such as nanowires (NWs), nanorods, nanotubes, and nanobelts of various materials in the past decade has prompted a need for methods to characterize their unique mechanical properties. These one-dimensional (1D) nanostructures possess superior mechanical properties [1, 2]; hence, applications of these structures ranging from nanoelectromechanical systems (NEMS) [3] to nanocomposites [4] are envisioned. Two overarching questions have spurred the development of nanomechanical testing techniques and the modeling of materials behavior at the nanoscale: how superior is material behavior at the nanoscale as compared to its bulk counterpart, and what are the underlying mechanisms that dictate this? Due to the limited number of atoms present in these nanostructures, they provide an excellent opportunity to couple experimentation and atomistic modeling on a one-to-one basis. This approach has the potential to greatly advance our understanding of material deformation and failure, as well as to validate the various assumptions employed in multiscale models proposed in the literature. A large number of atomistic simulations have been performed to predict nanostructure properties and reveal their deformation mechanisms [2]. However, due to the minute scale of these nanostructures, it has proven quite challenging to conduct well-instrumented mechanical testing and validate computational predictions. Earlier nanomechanical testing techniques include thermally-or electrically induced vibration of cantilevered nanostructures inside a transmission electron microscope (TEM) [5, 6]; lateral bending of suspended nanostructures using an atomic force microscope (AFM) [7, 8]; radial compression, by means of atomic force microscopy (AFM) probes, or nanoindentation of nanostructures on substrates [9, 10]; and tensile testing of freestanding nanostructures between two AFM cantilevers within a scanning electron microscope (SEM) [11]. Despite the exciting progress achieved by these methods, they are in general not well controlled in terms of loading, boundary conditions, and force-displacement measurements. In some instances, the nanostructure properties must be inferred from assumed models of the experimental setup. Recent advances in mechanical characterization of thin films have been remarkable and provide good insight for the testing of nanostructures. Among numerous techniques, two categories are particularly fascinating: in situ testing and on-chip testing. In situ testing provides a powerful means to obtain the deformation field and to observe the deformation mechanisms though real-time imaging, for example, by SEM. The SEM chamber is large enough to accommodate a microscale testing setup, and has been used for in situ tensile testing [12,13]. Another example of in situ testing involves an AFM to record the surface profile during a tensile test [14]. TEM is ideal for in situ testing since it provides direct evidence of the defects nucleation and reaction. Although most in situ TEM setups do not measure or control stresses in the specimen [15, 16], Haque and Saif recently incorporated a load sensor in the TEM [17, 18]. An on-chip testing system consists of micromachined elements, such as comb-drive actuators and force (load) sensors that can be integrated on a chip. One of the early attempts used electrostatic actuation and sensing for fatigue testing of silicon cantilever beams [19]. Osterberg and Senturia used electrostatic actuation for chip-level testing of cantilever beams, fixed-fixed beams, and clamped circular diaphragms to extract material properties [20]. Kahn et al. [21] used electrostatic actuators integrated with microfracture specimens to study fracture properties of polysilicon films. Owing to the capability of generating and measuring small-scale forces and displacements with high resolution, on-chip testing has the potential to impact the small-scale testing field profoundly. Note that these two concepts, in situ and on-chip, are related but different. On-chip testing can be performed in situ or ex situ. In situ testing does not necessarily utilize an on-chip device. But due to the small size of on-chip devices and integrated loading and force sensing capabilities, they conveniently facilitate in situ testing. In this chapter, the nanomechanical characterization of 1D nanostructures is reviewed. In Sect. 11.2, we summarize the challenges for mechanical characterization of 1D nanostructures. In Sect. 11.3, an overview of the existing experimental techniques is presented. In Sect. 11.4, a newly developed nanoscale material testing system for characterizing nanostructures is described in detail. In Sect. 11.5, some experimental results [22, 23] are summarized with emphasis on the in situ electromechanical testing of nanostructures, which complements our nanomechanical testing. © Springer Science+Business Media, LLC, 2008.