3 research outputs found
X-ray spectroscopy of electronic band structure in vanadium oxide nanoparticles
In order to elucidate the effects of nanostructuring on electron behavior in vanadium oxides, a suite of x-ray spectroscopy techniques was employed to comprehensively characterize the electronic structures of V2O5 and VO2 nanoparticles and compare them to their bulk counterparts. V2O5 and VO2 nanoparticle powders were characterized via PXRD, TEM, and HR-TEM to confirm size, purity, and crystallinity. Additionally, DSC and temperature-varied PXRD measurements on both VO2 samples confirmed the structural aspect of the monoclinic to rutile metal-insulator phase transition, and UV-Vis measurements allowed for Kubelka-Munk analysis on the V2O5 samples. XAS measurements enable the comparison of unoccupied conduction band states, while XES and RIXS measurements reveal occupied valence band states and the individual vanadium and oxygen PDOS below the Fermi level. XPS measurements of both core and valence band states both confirmed the valence band structure revealed by XES and also provide information on core-state energy levels. In the case of V2O5, the valence band O 2p states are upshifted in the nanoparticle sample, while the lowest V 3d conduction band states are unshifting but provide more available unoccupied states for excitation. These changes produce a shrunken bandgap in the V2O5 nanoparticles that is in line with much previous computational work, but unexpected from previous experimental results and defies the Moss-Burstein effect usually observed in V2O5. The resulting changes in band structure are attributed to a higher concentration of oxygen vacancy defects in the nanoparticle sample. Additionally, electron correlation effects in V2O5 nanoparticles are found to be enhanced relative to the bulk, likely due to added electron presence in the V 3d split-off band. In the case of VO2, dramatic changes in both the valence band and conduction band states are observed both below and above the structural phase transition temperature. These changes (lowered unoccupied conduction band states coupled with broadened and upshifted occupied valence band states) also lead to nanoparticle bandgap reduction and enhanced metallicity. The enhanced metallic nature of the VO2 nanoparticles is again attributed to the increased presence of surface oxygen vacancy defects, as well as a V2O3-like surface reconstruction. Additionally, electron correlation effects are found to be reduced in the VO2 nanoparticle samples relative to the bulk, unlike in the case of V2O5
Efficient, designable, and broad-bandwidth optical extinction via aspect-ratio-tailored silver nanodisks
Subwavelength resonators, ranging from single atoms to metallic
nanoparticles, typically exhibit a narrow-bandwidth response to optical
excitations. We computationally design and experimentally synthesize tailored
distributions of silver nanodisks to extinguish light over broad and varied
frequency windows. We show that metallic nanodisks are two-to-ten-times more
efficient in absorbing and scattering light than common structures, and can
approach fundamental limits to broadband scattering for subwavelength
particles. We measure broadband extinction per volume that closely approaches
theoretical predictions over three representative visible-range wavelength
windows, confirming the high efficiency of nanodisks and demonstrating the
collective power of computational design and experimental precision for
developing new photonics technologies