Pressure and Photoinduced Phase Transitions of Elemental Sulfur Confined by Open-End Single-Wall Carbon Nanohorns

Confined molecules in tubular nanospaces of nanocarbons, for example, carbon nanotubes and nanohorns (CNHs), lead to extraordinary behavior and properties different from their bulk analogues. Here, we investigated the confinement effect of CNHs on high-pressure elastic and vibrational properties of sulfur via the diamond anvil cell technique. X-ray diffraction measurements up to 40 GPa demonstrate two phase transitions of S-I → amorphous → S-II. A fit of equation of state yields a bulk modulus of ∼24.8 GPa, about 70% higher than that of soft sulfur. Different from previous Raman studies, laser with red light wavelength (694.8 nm) and high laser density (∼2 mW μm^-2) was employed under the threshold for generating C-S bonds. We observed a similar photoinduced transition of S-I to amorphous sulfur at 4-6 GPa compared to the results taken from blue and green light excitation and low laser density (e.g., <28 μW μm^-2), showing enhanced photothermal stability of sulfur by the aid of single-wall CNHs.