ABSTRACT
INTRODUCTION The second part of the 20th century has seen significant developments in our understanding of fundamental material science, and thus also of the mechanical performance of materials. This understanding has generated profound changes in the field, leading to new families of materials, new concepts, and wide-ranging improvements in the mechanical behavior and in all other properties of materials. In our energy-conscious society, materials and structures are required to be more performant, lightweight, and cheap. The best answer to these requirements is often provided through the powerful concept of reinforcement of a “matrix” material with second-phase dispersion (clusters, fibers). It is an interesting fact that many natural forms of reinforcement possess a nanometric dimension, whereas most current synthetic composites include fibers in the micrometer range. Expected benefits of such “miniaturization” would range from a higher intrinsic strength of the reinforcing phase (and thus of the composite) to more efficient stress transfer, to possible new and more flexible ways of designing the mechanical properties of yet even more advanced composites (1). Presently, reinforcement of common materials (alloys, polymers) with nanostructures is one of the most promising areas of study. As one of the major factors that determine the quality of reinforcement is the mechanical strength of nanostructures, the studies of elastic properties of nanomaterials are of significant importance. Besides reinforcement, investigation of the mechanical properties of nanowires is essential to determine the material strength for practical implementation as electronic or optical interconnects, as components in microelectromechanics, and as active or passive parts in nanosensors. Mechanical failure of those interconnects or building blocks may lead to malfunction, or even fatal failure of the entire device. Mechanical reliability, to some extent, will determine the long-term stability and performance for many of the nanodevices currently being designed and fabricated. When nanowire properties have been adequately explored and understood, their incorporation into solutions of practical problems will become evident more quickly and feasible for active and concerted pursuit. Nanomechanical measurements are a challenge, but remain essential to the fabrication, manipulation, and development of nanomaterials and perhaps even more so to our fundamental understanding of nanostructures. For this purpose, various experimental techniques, or methods, have been developed in the last several years, including tensile, resonance, nanoindentation, and bending tests. Traditional optical microscopy lacks the resolution to investigate phenomena of colloidal dimensions adequately, and electron and X-ray techniques are greatly limited either by environmental (e.g., liquids) or material property (e.g., conductivity, cross-section
for energy beam interactions) restrictions. Today, very few electron microscopes are capable of the true atomic resolution required for fundamental studies on intermolecular and colloidal behavior of two-or three-body interactions, for example. But scanning electron microscopes (SEMs) have played a crucial role in the study of mechanics with nanowires and nanosprings, and high-resolution optical microscopy is useful for locating these so-called one-dimensional objects on test substrates. In many cases, this is feasible because of lengths exceeding a few microns that scatter enough light for adequate contrast. The advent of atomic force microscopy (AFM) marked the beginning of significant advancement toward more routine molecular-scale imaging in three quantified dimensions with the simultaneous measurement of additional (one or more) physical properties. Researchers employ the AFM in various ways to determine sample mechanical properties, especially the elastic, or Young’s, modulus. Bending tests with AFM are common for mechanical characterization of nanowire-like systems owing to high spatial resolution and direct force measuring sensitivity.
