The
exceptional physical properties of graphene have sparked tremendous
interests toward two-dimensional (2D) materials with honeycomb structure.
We report here the successful fabrication of 2D iron tungstate (FeWOx) layers with honeycomb geometry on a Pt(111)
surface, using the solid-state reaction of (WO3)3 clusters with a FeO(111) monolayer on Pt(111). The formation process
and the atomic structure of two commensurate FeWOx phases, with (2 × 2) and (6 × 6) periodicities,
have been characterized experimentally by combination of scanning
tunneling microscopy (STM), low-energy electron diffraction (LEED),
X-ray photoelectron spectroscopy (XPS), and temperature-programmed
desorption (TPD) and understood theoretically by density functional
theory (DFT) modeling. The thermodynamically most stable (2 ×
2) phase has a formal FeWO3 stoichiometry and corresponds
to a buckled Fe2+/W4+ layer arranged in a honeycomb
lattice, terminated by oxygen atoms in Fe–W bridging positions.
This 2D FeWO3 layer has a novel structure and stoichiometry
and has no analogues to known bulk iron tungstate phases. It is theoretically
predicted to exhibit a ferromagnetic electronic ground state with
a Curie temperature of 95 K, as opposed to the antiferromagnetic behavior
of bulk FeWO4 materials.
Polarized and depolarized spectra from gold (Au) nanoparticles of different sizes are investigated in the small size range, between 3 and 7 nm, using low frequency Raman spectroscopy. Acoustic vibrations of the free-standing Au nanoparticles are demonstrated with frequencies ranging from 5 to 35 cm−1, opening the way to the development of the acoustic resonators. A blue shift in the phonon peaks along with the broadening is observed with a decrease in particle size. Comparison of the measured frequencies with vibrational dynamics calculation and an examination as from the transmission electron microscopy results ascertain that the low frequency phonon modes are due to acoustic phonon quantization. Our results show that the observed low frequency Raman scattering originates from the spherical (l = 0) and quadrupolar (l = 2) vibrations of the spheroidal mode due to plasmon mediated acoustic vibrations in Au nanoparticles.
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