Semiconductor nanowires, by definition, typically have nanoscale cross-sectional dimensions, with lengths spanning from hundreds of nanometers to millimeters. These subwavelength structures represent a new class of semiconductor materials for investigating light generation, propagation, detection, amplification, and modulation. After more than a decade of research, nanowires can now be synthesized and assembled with specific compositions, heterojunctions, and architectures. This has led to a host of nanowire photonic and electronic devices, including photodetectors, chemical and gas sensors, waveguides, LEDs, microcavity lasers, and nonlinear optical converters. Nanowires also represent an important class of nanostructure building blocks for photovoltaics as well as direct solar-to-fuel conversion because of their high surface area, tunable bandgap, and efficient charge transport and collection. This article gives a brief history of nanowire research for the past two decades and highlights several recent examples in our lab using semiconductor nanowires and their heterostructures for the purpose of solar energy harvesting and waste heat recovery.

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.

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.

Fundamental aspects of the interactions between cells and biomaterials provide a crucial framework for the design of many systems that interact with the human body. A comprehensive understanding of multifactorial processes may provide an adequate basis for the rational design of implantable devices ranging from nanoparticles for drug delivery and scaffolds for tissue engineering, to artificial organs for augmentation and rehabilitation. Recent progress in the elucidation of cell-biomaterials interactions has been predicated on advances in polymer chemistry, materials engineering, and device microfabrication. The confluence of these developments has stimulated a newfound ability to design soft materials and interfaces with precise chemical, physical, and mechanical properties. While static surfaces can yield insight into the interaction of mammalian cells with medical materials, the study of fundamental cell-biomaterials phenomena will benefit significantly from the ability to present biologically active signals to cells with spatiotemporal control. Specifically, the use of dynamic materials and interfaces can mirror the intrinsic dynamic behavior of living cells. This article highlights recent advances in soft materials design, interfacial engineering, and synthetic polymer networks in the context of producing dynamic materials to study cell-biomaterials interactions. Emerging challenges and future research directions are also discussed.

Self-folding of thin films is a more deterministic form of self-assembly wherein structures curve or fold up either spontaneously on release from the substrate or in response to specific stimuli. From an intellectual standpoint, the study of the self-folding of thin films at small size scales is motivated by the observation that a large number of naturally occurring materials such as leaves and tissues show curved, wrinkled, or folded micro- and nanoscale geometries. From a technological standpoint, such a self-assembly methodology is important since it can be used to transform the precision of existing planar patterning methods, such as electron-beam lithography, to the third dimension. Also, the self-folding of graphene promises a means to create a variety of three-dimensional carbon-based micro- and nanostructures. Finally, stimuli responsive self-folding can be used to realize chemomechanical and tether-free actuation at small size scales. Here, we review theoretical and experimental aspects of the self-folding of metallic, semiconducting, and polymeric films.

Thermal conductivity is a familiar property of materials: silver conducts heat well, and plastic does not. In recent years, an interdisciplinary group of materials scientists, engineers, physicists, and chemists have succeeded in pushing back long-established limits in the thermal conductivity of materials. Carbon nanotubes and graphene are at the high end of the thermal conductivity spectrum due to their high sound velocities and relative lack of processes that scatter phonons. Unfortunately, the superlative thermal properties of carbon nanotubes have not found immediate application in composites or interface materials because of difficulties in making good thermal contact with the nanotubes. At the low end of the thermal conductivity spectrum, solids that combine order and disorder in the random stacking of two-dimensional crystalline sheets, so-called “disordered layered crystals,” show a thermal conductivity that is only a factor of 2 larger than air. The cause of this low thermal conductivity may be explained by the large anisotropy in elastic constants that suppresses the density of phonon modes that propagate along the soft direction. Low-dimensional quantum magnets demonstrate that electrons and phonons are not the only significant carriers of heat. Near room temperature, the spin thermal conductivity of spin-ladders is comparable to the electronic thermal conductivities of metals. Our measurements of nanoscale thermal transport properties employ a variety of ultrafast optical pump-probe metrology tools that we have developed over the past several years. We are currently working to extend these techniques to high pressures (60 GPa), high magnetic fields (5 T), and high temperatures (1000 K).