Metastable Group IV Allotropes and Solid Solutions: Nanoparticles and Nanowires

In the past decades, group IV nanowires and nanoparticles have been the subject of extensive research. Beside tremendous progress in morphological control and integration in advanced device architectures, research on allotropes and metastable compositions has gained considerable interest. Several new approaches now allow the controlled formation of specific allotropes in the nanostructured form as well as chemical compositions not attainable by traditional synthesis protocols. The conditions applied to form these metastable solid solutions and allotropes are usually far from thermodynamic equilibrium or rely on unconventional templates. The increased interest in the field of metastable group IV nanostructures arises from their altered physical properties, including tunable, direct bandgaps with energies equivalent to the near- to mid-infrared spectral region as described for materials such as Ge_1–xSn_x and hexagonal Si_1–xGe_x. The implementation of these material characteristics in complementary metal oxide semiconductor (CMOS) processes are desirable for applications in electronics, optoelectronics, sensors, optics, etc. but also their use as nonsurface bound nanoparticles in sensing, nanobiotechnology and nanomedicine can offer additional opportunities. This review article highlights both the important advancements and still open questions for the continued development of these nanoscaled materials for next-generation device concepts.