Biogenic minerals often contain inorganic and organic impurities that are believed to harden and toughen the material. However, because of the complexity of these systems, it is difficult to deconvolute the effect of each of these impurities on the hardness of the material. We have created single-crystal samples with a range of magnesium concentrations and measured their hardness while controlling for orientation. We find that hardness increases linearly with magnesium content and that magnesium impurities could account for ~20% of the increased hardness in biogenic calcite from the mollusk Atrina rigida when compared with pure geologic calcite.

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).

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.

Development of 3C–SiC-based electronics is hampered by film quality and wafer bow produced during growth on silicon. This work presents an approach aimed to improve the compliance between Si and 3C–SiC by manipulating Si substrate surface with the creation of an array of squared-base Inverted Silicon Pyramids (ISP) and stimulating the annihilation of defects created at the interface. A reduction of stacking fault (SF) linear density to a value of 9.31 × 103 cm−1 has been observed in 9-μm-thick 3C–SiC film on ISP, stimulated by the ISP geometry with SFs forced to meet one another within the first micrometer of growth. The initial growth of 3C–SiC on ISP is described suggesting a peculiar growth mode leading to uniform sample morphology after about 3 μm of growth. Finally, lower residual stress stored in 3C–SiC/ISP samples has been observed, due to a faster stress relaxation mechanism during the film growth.

A functionalization method for the specific and selective immobilization of the streptavidin (SA) protein on semiconductor nanowires (NWs) was developed. Silicon (Si) and silicon carbide (SiC) NWs were functionalized with 3-aminopropyltriethoxysilane (APTES) and subsequently biotinylated for the conjugation of SA. Existence of a thin native oxide shell on both Si and SiC NWs enabled efficient binding of APTES with the successive attachment of biotin and SA as was confirmed with x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and atomic force microscopy. Fluorescence microscopy demonstrated nonspecific, electrostatic binding of the SA and the bovine serum albumin (BSA) proteins to APTES-coated NWs. Inhibition of nonspecific BSA binding and enhancement of selective SA binding were achieved on biotinylated NWs. The biofunctionalized NWs have the potential to be used as biosensing platforms for the specific and selective detection of proteins.