Stable optical trapping and sensitive characterization of nanostructures using standing- wave Raman tweezers

Optical manipulation and label-free characterization of nanoscale structures open up new possibilities for assembly and control of nanodevices and biomolecules. Optical tweezers integrated with Raman spectroscopy allows analyzing a single trapped particle, but is generally less effective for individual nanoparticles. The main challenge is the weak gradient force on nanoparticles that is insufficient to overcome the destabilizing effect of scattering force and Brownian motion. Here, we present standing-wave Raman tweezers for stable trapping and sensitive characterization of single isolated nanostructures with a low laser power by combining a standing-wave optical trap with confocal Raman spectroscopy. This scheme has stronger intensity gradients and balanced scattering forces, and thus can be used to analyze many nanoparticles that cannot be measured with single-beam Raman tweezers, including individual single-walled carbon nanotubes (SWCNT), graphene flakes, biological particles, SERS-active metal nanoparticles, and high-refractive semiconductor nanoparticles. This would enable sorting and characterization of specific SWCNTs and other nanoparticles based on their increased Raman fingerprints

Formation of filamentous carbon through dissociation of chromium carbide under hydrothermal conditions

Synthesis of filamentous carbon through the decomposition of chromium carbide was studied employing hydrothermal technique in the pressure and temperature range of 100–200 MPa and 350–800 °C respectively. It was found that chromium carbide dissociates into chromium oxide in the presence of water at temperature <400 °C. But, the formation of free elemental carbon as filamentous particles was noticed in the presence of organic compounds at temperatures above 600 °C. The organic compounds are known to dissociate to C–O–H supercritical fluids under hydrothermal condition. The supercritical fluids generated by the dissociation of organic compounds have great influence on the decomposition of chromium carbide. The scanning electron microscopic (SEM) studies of the experimental run products show that the fibrous or filamentous form of carbon was found with a few spherical shaped carbons, in the chromium carbide—organic compound runs. These carbon particles were solid curved filaments with a mean diameter of 50–100 nm. Micro Raman spectroscopic studies show that the filaments have sp 2 -hybridized carbon atoms