Developing wireless nanodevices and nanosystems is critical for sensing, medical science, environmental/infrastructure monitoring, defense technology, and even personal electronics. It is highly desirable for wireless devices to be self-powered without using a battery. We have developed piezoelectric nanogenerators that can serve as self-sufficient power sources for micro-/nanosystems. For wurtzite structures that have non-central symmetry, such as ZnO, GaN, and InN, a piezoelectric potential (piezopotential) is created by applying a strain. The nanogenerator uses the piezopotential as the driving force, responding to dynamic straining of piezoelectric nanowires. A gentle strain can produce an output voltage of up to 20–40 V from an integrated nanogenerator. Furthermore, piezopotential in the wurtzite structure can serve as a “gate” voltage that can effectively tune/control charge transport across an interface/junction; electronics based on such a mechanism are referred to as piezotronics, with applications such as electronic devices that are triggered or controlled by force or pressure, sensors, logic units, and memory. By using the piezotronic effect, we show that optoelectronic devices fabricated using wurtzite materials can provide superior performance for solar cells, photon detectors, and light-emitting diodes. Piezotronic devices are likely to serve as “mediators” for directly interfacing biomechanical action with silicon-based technology. This article reviews our study of ZnO nanostructures over the last 12 years, with a focus on nanogenerators and piezotronics.

Thermal-barrier coatings (TBCs) are complex, defected, thick films made of zirconia-based refractory ceramic oxides. Their widespread applicability has necessitated development of high throughput, low cost materials manufacturing technologies. Thermal plasmas and electron beams have been the primary energy sources for processing of such systems. Electron-beam physical vapor deposition (EBPVD) is a sophisticated TBC fabrication technology for rotating parts of aero engine components, while atmospheric plasma sprays (APS) span the range from rotating blades of large power generation turbines to afterburners in supersonic propulsion engines. This article presents a scientific description of both contemporary manufacturing processes (EBPVD, APS) and emerging TBC deposition technologies based on novel extensions to plasma technology (suspension spray, plasma spray-PVD) to facilitate novel compliant and low thermal conductivity coating architectures. TBCs are of vital importance to both performance and energy efficiency of modern turbines with concomitant needs in process control for both advanced design and reliable manufacturing.

Papyrus writings from 1600–1500 BC describe cancer and the attempts at treatment. Centuries later, cancer remains a devastating disease. Given the long history of difficulties in developing cancer therapies, why is there excitement about nanoparticle medicine (nanomedicines) for fighting cancer? This article describes the current understanding of why these engineered, nano-sized medicines, which are highly multifunctional chemical systems, have the potential to provide revolutionary ways to treat cancer. This point is illustrated by physical insights at the nanoscale that allow for the development of nanoparticles that can function in both animals and humans. The human data show how we have translated two independent nanoparticle cancer therapeutics from laboratory curiosities to experimental therapeutics in human clinical trials.

We present an introduction to the rapidly growing field of epitaxial graphene on silicon carbide, tracing its development from the original proof-of-concept experiments a decade ago to its present, highly evolved state. The potential of epitaxial graphene as a new electronic material is now being recognized. Whether the ultimate promise of graphene-based electronics will ever be realized remains an open question. Silicon electronics is based on single-crystal substrates that allow reliable patterning on the nanoscale, which is an absolute requirement for any new electronic material. That is why epitaxial graphene is based on single-crystal silicon carbide. We also present recent results on nanopatterned graphene produced by etching the silicon carbide before annealing so that the graphene structures are produced in their final shapes. This avoids postannealing patterning, which is known to greatly affect transport properties on the nanoscale. Creating such structured graphene is an elegant method for avoiding pervasive patterning problems.

Scanning probe microscopy (SPM) has made significant contributions to our understanding of the sub-processes underlying photovoltaic action. These techniques allow local investigation of the electrical and optical properties of a material. Spatially resolved measurements of surface photovoltage and photocurrent have been particularly useful in understanding charge generation and separation. In thin-film inorganic solar cells, for example, Kelvin probe force microscopy (KPFM) has revealed that charge separation does not occur at a heterojunction as expected, but instead occurs at a homojunction buried ∼50 nm within the absorbing layer. In organic photovoltaics, submicron maps of photocurrent have contributed to the understanding of the interplay between processing conditions, blend morphology, and device performance. Such functional imaging distinguishes SPM from complementary structural characterization techniques. Our goal in this article is to provide the materials science community with an appreciation for the capabilities, considerations, and limitations associated with SPM studies of solar cell materials and devices. Highlighted techniques include scanning tunneling microscopy, photoconductive atomic force microscopy, near-field scanning optical microscopy, KPFM, and time-resolved electric force microscopy.

Phase change materials can be switched rapidly and repeatedly between amorphous and crystalline phases, which differ distinctly in their optical and electrical properties. This combination of properties is utilized to store information in rewritable optical storage media and in emerging phase change memory technology. This article describes the physical properties of phase change materials such as Ge2Sb2Te5 and relates these properties to specific structural and bonding characteristics. Electrical conduction and switching, which are relevant for phase change memory operation, are explained from a physical perspective. Phase change memory device integration and technology development are discussed, including aspects of access device selection and integration.

Emerging telecommunication and data routing applications anticipate a photonic roadmap leading to ultra-compact photonic integrated circuits. Consequently, photonic devices will soon have to meet footprint and efficiency requirements similar to their electronic counterparts calling for extreme capabilities to create, guide, modulate, and detect deep-subwavelength optical fields. For active devices such as modulators, this means fulfilling optical switching operations within light propagation distances of just a few wavelengths. Plasmonics, or metal optics, has emerged as one potential solution for integrated on-chip circuits that can combine both high operational speeds and ultra-compact architectures rivaling electronics in both speed and critical feature sizes. This article describes the current status, challenges, and future directions of the various components required to realize plasmonic integrated circuitry.

This article discusses some of the many-body potentials used for simulations of processes and energies in materials at the atomic scale, emphasizing their motivation and underlying physical concepts, particularly where these are not entirely empirical. The perspective is somewhat historical and describes the importance of developments of the theory of electrons in solids for the derivation of many-body (or many-atom) potential models. The models include density-dependent pairwise potentials, effective medium and embedded-atom models, and polarizable ion models. As a recent radical departure from approaches derived from the physics of electrons, the development of models based on information theory is also described.

In the emerging field of soft machines, large deformation of soft materials is harnessed to provide functions such as regulating flow in microfluidics, shaping light in adaptive optics, harvesting energy from ocean waves, and stretching electronics to interface with living tissues. Soft materials, however, do not provide all of the requisite functions; rather, soft machines are mostly hybrids of soft and hard materials. In addition to requiring stretchable electronics, soft machines often use soft materials that can deform in response to stimuli other than mechanical forces. Dielectric elastomers deform under a voltage. Hydrogels swell in response to changes in humidity, pH, temperature, and salt concentration. How does mechanics meet geometry, chemistry, and electrostatics to generate large deformation? How do molecular processes affect the functions of transducers? How efficiently can materials convert energy from one form to another? These questions are stimulating intriguing and useful advances in mechanics. This review highlights the mechanics that enables the creation of soft machines.

As biological science advances, there is a need for new technical tools to study biological matters. In neuroscience, new knowledge on the nervous system is discovered through biological experiments carried out under in vitro conditions. As experiments become more delicate, the technical requirements also increase. Recent advances in nano- and microscale technologies have increased the applicability of new emerging technology to neurobiology and neural engineering. As a result, many materials that were not originally developed for neural interfaces have become attractive candidates to sense neural signals, stimulate neurons, and grow nerve cells for tissue engineering. This article focuses on the material requirements for in vitro neural interfaces and introduces materials that are used to design various neural interface platforms in vitro.

By measuring the energy losses of high-energy electrons transmitted through a thin sample, electron energy-loss spectroscopy provides information on the local electronic structure in materials. Using electron beams smaller than 0.1 nm, the technique provides exquisite sensitivity to changes in valence and coordination of the excited atoms such that local changes in the bonding environment are probed with a resolution approaching the Ångstrøm level, with an energy resolution competitive with complementary techniques such as x-ray absorption spectroscopy. With the development of spectroscopic imaging in the scanning transmission electron microscope, this technique can be used to map, at the atomic level, the composition of atomic columns and the valence of atoms at defects, interfaces, and surfaces. Recent applications of this technique are provided as examples showing the potential of the method for materials research.

Microelectromechanical systems (MEMS) incorporating piezoelectric layers provide active transduction between electrical and mechanical energy, which enables highly sensitive sensors and low-voltage driven actuators surpassing the passive operation of electrostatic MEMS. Several different piezoelectric materials have been successfully integrated into MEMS structures, most notably Pb(Zr,Ti)O3. Piezoelectric materials with larger piezoelectric response, such as the relaxor ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3(PMN-PT), would enable further miniaturization. However, this has long been hampered by the difficulties in the synthesis of these materials. This article reviews recent successes not only in synthesizing high-quality epitaxial PMN-PT heterostructures on Si, but also in fabricating PMN-PT microcantilevers, which retain the piezoelectric properties of bulk PMN-PT single crystals. These epitaxial heterostructures provide a platform to build MEMS and nanoelectromechanical system devices that function with large displacement at low drive voltages, such as ultrasound medical imagers, micro-fluidic control, piezotronics, and energy harvesting.

The success of the correction of spherical aberration in the electron microscope has revolutionized our view of oxides. This is a very important class of materials that is promising for future applications of some of the most intriguing phenomena in condensed matter physics: colossal magnetoresistance, colossal ionic conductivity, high Tc superconductivity, and ferroelectricity. Understanding the physics underlying such phenomena, especially in low dimensional systems (thin films, interfaces, nanowires, nanoparticles), relies on the availability of techniques capable of looking at these systems in real space and with atomic resolution and even beyond, with single atom sensitivity; in many cases, the system properties depend on minuscule amounts of point defects that alter the material’s properties dramatically. Atomic resolution spectroscopy in the aberration-corrected electron microscope is one of the most powerful techniques available to materials scientists today. This article will briefly review some state-of-the-art applications to oxide materials: from atomic resolution elemental mapping and single atom imaging to applications to real systems, including oxide interfaces and mapping of physical properties such as the spin state of magnetic atoms.

A range of material systems exist in which nanoscale ionic transport and redox reactions provide the essential mechanisms for memristive switching. One class relies on mobile cations, which are easily created by electrochemical oxidation of the corresponding electrode metal, transported in the insulating layer, and reduced at the inert counterelectrode. These devices are termed electrochemical metallization (ECM) memories, also called conductive bridge random access memories. The memristive characteristics of the ECM cells provide opportunities for circuit design and computational concepts that go beyond those in traditional complementary metal oxide semiconductor (CMOS) technology. Passive memory arrays open up paths toward ultradense and 3D stackable memory and logic gate arrays. Furthermore, the multivalued conductance characteristics allow for potential exploitation of the cells as synapses in neuromorphic circuits in future energy efficient high-performance computer architectures. Despite exciting results obtained in recent years, many challenges have to be met before these physical effects can be turned into competitive industrial technology. Here, we briefly review the basic working principle, the different possible and potential material combinations, and the fundamental electrochemical processes in ECM cells and their implications for device operations. The prospects of ECM-based resistive random access memory as an emerging memory technology are also reviewed in terms of switching speed and scalability.

Thermal-barrier coatings are complex systems with properties that largely depend on their specific microstructure. Their properties change during operation, typically leading to degradation. A further difficulty arises from the fact that this degradation also depends on specific loading conditions that can be rather complex. Different laboratory setups are described that simulate, at least partially, the actual loading conditions. In addition, sensing and nondestructive methods are described that are targeted toward reliable operation of a gas-turbine engine with thermal-barrier coated components.

Epitaxial graphene (EG) has attracted considerable interest because of its extraordinary properties and ability to be synthesized on the wafer scale. These attributes have enabled EG to be applied in field-effect transistors with extrinsic operating frequencies in the hundreds-of-gigahertz range. Although the quality of EG grown on SiC has improved, there are still obstacles, such as low carrier mobility and large-area thickness nonuniformity, that limit applications in a wide range of truly wafer-scale technologies. In this article, key elements of epitaxial graphene synthesis are highlighted and discussed with regard to impacts on large-area uniformity, structure, and electrical properties. The effects of specific components such as growth-reactor design and substrate quality are examined in an effort to provide a pathway for future advancements in EG production. Finally, key future directions for research in EG are briefly discussed.