Coupling between mechanical and electrical phenomena is ubiquitous at the nano-and molecular scales, with examples ranging from piezoelectricity and flexoelectricity in perovskites to complex molecular transformations in redox active molecules and ion channels. This article delineates the field of nanoelectromechanics enabled by recent advances in scanning probe, indentation, and interferometric techniques and provides a unified outlook at a number of related topics, including membrane and surface flexoelectricity, local piezoelectricity in ferroelectrics and associated devices, and electromechanical molecular machines. It also summarizes experimental and theoretical challenges on the pathway to visualize, control, and manipulate electromechanical activity on the nanoscale and molecular levels.

In the past two decades, the fact that “small is different” has been established for a wide variety of phenomena, including electrical, optical, magnetic, and mechanical behavior of materials. However, one largely untapped but potentially very important area of nanoscience involves the interplay of electricity and mechanics at the nanoscale. In this article, predicated on both phenomenological approaches and atomistic calculations, we summarize the state-of-the-art in understanding electromechanical coupling at the nanoscale. First, we address flexoelectricity—the coupling of strain gradient to polarization. Flexoelectricity exists in both piezoelectric and nonpiezoelectric dielectrics. As a high-order spatial-dispersion effect, the flexoelectricity becomes more and more important with the reduction of the spatial scale of the problem. Exploitation of this phenomenon and the associated nanoscale size effects can lead to tantalizing applications, such as “piezoelectric nanocomposites without using piezoelectric materials.” The second issue concerns electromechanical effects at the dielectric/metal interface. An interface in solids typically exhibits a lower symmetry compared to that of the associated adhering materials. This symmetry reduction can drastically affect the electromechanical and dielectric behavior of the material at the nanoscale.

Piezoresponse force microscopy (PFM) is a powerful method widely used for nanoscale studies of the electromechanical coupling effect in various materials systems. Here, we review recent progress in this field that demonstrates great potential of PFM for the investigation of static and dynamic properties of ferroelectric domains, nanofabrication and lithography, local functional control, and structural imaging in a variety of inorganic and organic materials, including piezoelectrics, semiconductors, polymers, biomolecules, and biological systems. Future pathways for PFM application in high-density data storage, nanofabrication, and spectroscopy are discussed.

Piezoelectric microelectromechanical systems (MEMS) offer the opportunity for high-sensitivity sensors and large displacement, low-voltage actuators. In particular, recent advances in the deposition of perovskite thin films point to a generation of MEMS devices capable of large displacements at complementary metal oxide semiconductor-compatible voltage levels. Moreover, if the devices are mounted in mechanically noisy environments, they also can be used for energy harvesting. Key to all of these applications is the ability to obtain high piezoelectric coefficients and retain these coefficients throughout the microfabrication process. This article will review the impact of composition, orientation, and microstructure on the piezoelectric properties of perovskite thin films such as PbZr1−xTixO3 (PZT). Superior piezoelectric coefficients (e31, f of −18 C/m2) are achieved in {001}-oriented PbZr0.52Ti0.48O3 films with improved compositional homogeneity on Si substrates. The advent of such high piezoelectric responses in films opens up a wide variety of possible applications. A few examples of these, including low-voltage radio frequency MEMS switches and resonators, actuators for millimeter-scale robotics, droplet ejectors, energy scavengers for unattended sensors, and medical imaging transducers, will be discussed.

Cells are ion conductive gels surrounded by a ∼5-nm-thick insulating membrane, and molecular ionic pumps in the membrane establish an internal potential of approximately −90 mV. This electrical energy store is used for high-speed communication in nerve and muscle and other cells. Nature also has used this electric field for high-speed motor activity, most notably in the ear, where transduction and detection can function as high as 120 kHz. In the ear, there are two sets of sensory cells: the “inner hair cells” that generate an electrical output to the nervous system and the more numerous “outer hair cells” that use electromotility to counteract viscosity and thus sharpen resonance to improve frequency resolution. Nature, in a remarkable exhibition of nanomechanics, has made out of soft, aqueous materials a microphone and high-speed decoder capable of functioning at 120 kHz, limited only by thermal noise. Both physics and biology are only now becoming aware of the material properties of biomembranes and their ability to perform work and sense the environment. We anticipate new examples of this biopiezoelectricity will be forthcoming.

Recent developments in chemical synthesis, nanoscale assembly, and molecular-scale measurements enable the extension of the concept of macroscopic machines to the molecular and supramolecular levels. Molecular machines are capable of performing mechanical movements in response to external stimuli. They offer the potential to couple electrical or other forms of energy to mechanical action at the nano- and molecular scales. Working hierarchically and in concert, they can form actuators referred to as artificial muscles, in analogy to biological systems. We describe the principles behind driven motion and assembly at the molecular scale and recent advances in the field of molecular-level electromechanical machines, molecular motors, and artificial muscles. We discuss the challenges and successes in making these assemblies work cooperatively to function at larger scales.

Reticular chemistry concerns the linking of molecular building blocks into predetermined structures using strong bonds. We have been working on creating and developing the conceptual and practical basis of this new area of research. As a result, new classes of crystalline porous materials have been designed and synthesized: metal-organic frameworks, zeolitic imidazolate frameworks, and covalent organic frameworks. Crystals of this type have exceptional surface areas (2,000−6,000 m2/g) and take up voluminous amounts of hydrogen (7.5 wt% at 77 K and 3−4 × 106 Pa), methane (50 wt% at 298 K and 2.5 × 106 Pa), and carbon dioxide (140 wt% at 298 K and 3 × 106 Pa). We have driven the basic science all the way to applications without losing sight of our quest for understanding the underlying molecular aspects of this chemistry. The presentation was focused on the design concepts, synthesis, and structure of these materials, with emphasis on their applications to onboard energy storage.