Synthesis of well-ordered reduced dimensional carbon solids with extended bonding remains a challenge. For example, few single-crystal organic monomers react under topochemical control to produce single-crystal extended solids. We report a mechanochemical synthesis in which slow compression at room temperature under uniaxial stress can convert polycrystalline or single-crystal benzene monomer into single-crystalline packings of carbon nanothreads, a one-dimensional sp3 carbon nanomaterial. The long-range order over hundreds of microns of these crystals allows them to readily exfoliate into fibers. The mechanochemical reaction produces macroscopic single crystals despite large dimensional changes caused by the formation of multiple strong, covalent C–C bonds to each monomer and a lack of reactant single-crystal order. Therefore, it appears not to follow a topochemical pathway, but rather one guided by uniaxial stress, to which the nanothreads consistently align. Slow-compression room-temperature synthesis may allow diverse molecular monomers to form single-crystalline packings of polymers, threads, and higher dimensional carbon networks.

Semiconductor core optical fibers with a silica cladding are of great interest in nonlinear photonics and optoelectronics applications. Laser crystallization has been recently demonstrated for crystallizing amorphous silicon fibers into crystalline form. Here we explore the underlying mechanism by which long single-crystal silicon fibers, which are novel platforms for silicon photonics, can be achieved by this process. Using finite element modeling, we construct a laser processing diagram that reveals a parameter space within which single crystals can be grown. Utilizing this diagram, we illustrate the creation of single-crystal silicon core fibers by laser crystallizing amorphous silicon deposited inside silica capillary fibers by high-pressure chemical vapor deposition. The single-crystal fibers, up to 5.1 mm long, have a very well-defined core/cladding interface and a chemically pure silicon core that leads to very low optical losses down to ∼0.47–1 dB/cm at the standard telecommunication wavelength (1550 nm). It also exhibits a photosensitivity that is comparable to bulk silicon. Creating such laser processing diagrams can provide a general framework for developing single-crystal fibers in other materials of technological importance.Keywords: chemical vapor deposition; crystal growth; laser crystallization; optical fiber; optoelectronics; silicon photonics;

Polycrystalline silicon core optical fibers have been fabricated by modified thermal annealing of amorphous silicon chemically deposited at high pressure. The resulting fibers have small-diameter cores, a geometry advantageous for optical guidance. Moreover, the combination of chemical deposition and annealing avoids difficulties associated with undesired transfer of oxygen impurities to the silicon core from the molten cladding during the drawing process. The high aspect ratio of the amorphous silicon core and the presence of the silica cladding surrounding make the design rules for annealing to optimize their polycrystalline structure different from those of conventional amorphous silicon films. We find that optimization of the annealing allows for an increase in the polycrystalline grain size and decrease in the defects in the silicon core. A low optical loss of less than 1 dB/cm at a wavelength of 2.2 μm is thus realized, much lower than that reported for small core size (<10 μm) crystalline silicon fibers and comparable to the loss in many planar semiconductor waveguides. This loss is just below the threshold of 1 dB/cm often considered necessary for many photonic and optoelectronic applications at near to mid-infrared wavelengths in areas such as nonlinear photonics, lasers, and in-fiber photodetectors. Further reduction in optical losses as deposition and annealing techniques are improved can be anticipated.

Hydrogenated amorphous silicon (a-Si:H) is one of the most technologically important semiconductors. The challenge in producing it from SiH4 precursor is to overcome a significant kinetic barrier to decomposition at a low enough temperature to allow for hydrogen incorporation into a deposited film. The use of high precursor concentrations is one possible means to increase reaction rates at low enough temperatures, but in conventional reactors such an approach produces large numbers of homogeneously nucleated particles in the gas phase, rather than the desired heterogeneous deposition on a surface. We report that deposition in confined micro-/nanoreactors overcomes this difficulty, allowing for the use of silane concentrations many orders of magnitude higher than conventionally employed while still realizing well-developed films. a-Si:H micro-/nanowires can be deposited in this way in extreme aspect ratio, small-diameter optical fiber capillary templates. The semiconductor materials deposited have ∼0.5 atom% hydrogen with passivated dangling bonds and good electronic properties. They should be suitable for a wide range of photonic and electronic applications such as nonlinear optical fibers and solar cells.

Advances in nanoscience are critically dependent on the ability to control and probe chemical and physical phenomena in confined geometries. A key challenge is to identify confinement structures with high surface area to volume ratios and controlled surface boundaries that can be probed quantitatively at the molecular level. Herein we report an approach for probing molecular structures within nano- to microscale pores by the application of spontaneous Raman spectroscopy. We demonstrate the method by characterization of the structural features of picomole quantities of well-organized octadecyltrichlorosilane (OTS) monolayers self-assembled on the interior pore surfaces of high aspect ratio (1 μm diameter × 1−10 cm length), near-atomically smooth silica microstructured optical fibers (MOFs). The simple Raman backscattering collection geometry employed is well suited for a wide variety of diagnostic applications as demonstrated by tracking the combustion of the hydrocarbon chains of the OTS self-assembled monolayer (SAM) and spectral confirmation of the formation of an adsorbed monolayer of human serum albumin (HSA) protein. Using this MOF Raman approach, molecular processes in precisely defined, highly confined geometries can be probed at high pressures and temperatures, with a wide range of excitation wavelengths from the visible to the near-IR, and under other external perturbations such as electric and magnetic fields.

The ultraviolet (257 nm) Raman spectrum of C60 compressed to 30 GPa in a Mao–Bell diamond anvil cell with no pressure transmitting medium at ambient temperature indicates the formation of diamond after release of pressure. Previously, more extreme non-hydrostatic compression was reported to be required to form diamond from C60. These results provide confirmation of the transformation of C60 to diamond upon non-hydrostatic compression at room temperature and illustrate the utility of UV Raman spectroscopy for the analysis of carbon phases containing both sp2 and sp3 bonding.

The thermoelectric power (TEP) of Ni and Yb have been investigated up to 10 and 7 GPa, respectively, with a new laser-heated diamond anvil cell technique. Ni exhibits free electron-like behavior that can be explained by the Mott expression for thermopower. A small decrease in the TEP is observed in the pressure range 2–3 GPa. There are variations in the TEP of Yb as a function of pressure that correlate with previously observed structural phase transitions. The increase in TEP of Yb in the metallic bcc phase is understood from the increase in the Hubbard coulomb correlation energy.