The ability to produce small scale, crystalline silicon spheres is of significant technological and scientific importance, yet scalable methods for doing so have remained elusive. Here we demonstrate a silicon nanosphere fabrication process based on an optical fibre drawing technique. A silica-cladded silicon-core fibre with diameters down to 340 nm is continuously fed into a flame defining an axial thermal gradient and the continuous formation of spheres whose size is controlled by the feed speed is demonstrated. In particular, spheres of diameter o500 nm smaller than those produced under isothermal heating conditions are shown and analysed. A fibre with dual cores, p-type and n-type silicon, is drawn and processed into spheres. Spatially coherent break-up leads to the joining of the spheres into a bispherical silicon 'p-n molecule'. The resulting device is measured to reveal a rectifying I-V curve consistent with the formation of a p-n junction.

A new all-in-fiber trace-level chemical sensing approach is demonstrated. Photoconductive structures, embedded directly into the fiber cladding along its entire length, capture light emitted anywhere within the fiber's hollow core and transform it directly into an electrical signal. Localized signal transduction circumvents problems associated with conventional fiber-optics, including limited signal collection efficiency and optical losses. This approach facilitates a new platform for remote and distributed photosensing.

Crystallization of microdroplets of molten alloys could, in principle, present a number of possible morphological outcomes, depending on the symmetry of the propagating solidification front and its velocity, such as axial or spherically symmetric species segregation. However, because of thermal or constitutional supercooling, resulting droplets often only display dendritic morphologies. Here we report on the crystallization of alloyed droplets of controlled micrometer dimensions comprising silicon and germanium, leading to a number of surprising outcomes. We first produce an array of silicon−germanium particles embedded in silica, through capillary breakup of an alloy-core silica-cladding fiber. Heating and subsequent controlled cooling of individual particles with a two-wavelength laser setup allows us to realize two different morphologies, the first being a silicon−germanium compositionally segregated Janus particle oriented with respect to the illumination axis and the second being a sphere made of dendrites of germanium in silicon. Gigapascal-level compressive stresses are measured within pure silicon solidified in silica as a direct consequence of volume-constrained solidification of a material undergoing anomalous expansion. The ability to generate microspheres with controlled morphology and unusual stresses could pave the way toward advanced integrated in-fiber electronic or optoelectronic devices. multimaterial fibers | microparticles | confined solidification | silicon−germanium spheres | stressed silicon C ontrolling the microstructure or state of stress of microparticles and nanoparticles is often key to attaining the desired properties for a specific application (1-5); however, the ability to do so is strongly limited by the synthesis method. For instance, nonspherically symmetric distributions of inorganic materials are difficult to achieve from bottom-up approaches (4-7). Likewise, controlling the state of stress or strain of semiconductor particles is challenging in unconstrained nucleation-and-growth synthesis methods. However, Janus particles of silicon−germanium (SiGe) could potentially find applications as microswimmers or nanoswimmers owing to asymmetric absorption properties (8), as well as in infrared photodetectors or solar cells for increased infrared absorption (9). Stressed silicon particles, on the other hand, could be used for bandgap tunability in photonic or optoelectronic devices (10-12).In the past few years, thermally drawn multimaterial fibers have emerged as a unique platform for top-down scalable fabrication of microparticles to nanoparticles over a broad range of materials, through controlled in-fiber capillary breakup of the fiber components (13-15). In the case of polymers or chalcogenide glasses, structural control of the particle can be achieved by constructing complex cores at the preform level, which is later broken up in the fiber state to form structured particles (13). However, in the case of traditional semiconductor materials such as silicon and germanium, the same...