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Crystal Symmetry Breaking in Few-Quintuple Bismuth Telluride Films: Applications in Nanometrology of Topological Insulators
We report results of micro-Raman spectroscopy investigation of the
"graphene-like" mechanically exfoliated single-crystal bismuth telluride films
with the thickness ranging from a few-nm-range to bulk limit. It is found that
the optical phonon mode A1u, which is not-Raman active in bulk bismuth
telluride crystals, appears in the atomically-thin films due to
crystal-symmetry breaking. The intensity ratios of the out-of-plane A1u and A1g
modes to the in-plane Eg mode grow with decreasing film thickness. The
evolution of Raman signatures with the film thickness can be used for
identification of bismuth telluride crystals with the thickness of
few-quintuple layers, which are important for topological insulator and
thermoelectric applications.Comment: 13 pages, 2 tables, 3 figures; to be presented at MRS Spring Meeting,
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Preparation of high crystalline nanoparticles of rare-earth based complex pervoskites and comparison of their structural and magnetic properties with bulk counterparts
A simple route to prepare GdSrMnO nanoparticles by
ultrasonication of their bulk powder materials is presented in this article.
For comparison, GdSrMnO nanoparticles are also prepared by
ball milling. The prepared samples are characterized by X-ray diffraction
(XRD),field emission scanning electron microscope (FESEM), energy dispersive
X-ray (EDX), X-ray photoelectron spectroscope (XPS), and Superconducting
Quantum Interference Device (SQUID) magnetometer. XRD Rietveld analysis is
carried out extensively for the determination of crystallographic parameters
and the amount of crystalline and amorphous phases. FESEM images demonstrate
the formation of nanoparticles with average particle size in the range of
50-100 nm for both ultrasonication and 4 hours (h) of ball milling. The bulk
materials and nanoparticles synthesized by both ultrasonication and 4 h ball
milling exhibit a paramagnetic to spin-glass transition. However, nanoparticles
synthesized by 8 h and 12 h ball milling do not reveal any phase transition,
rather show an upturn of magnetization at low temperature. The degradation of
the magnetic properties in ball milled nanoparticles may be associated with
amorphization of the nanoparticles due to ball milling particularly for milling
time exceeding 8 h. This investigation demonstrates the potential of
ultrasonication as a simple route to prepare high crystalline rare-earth based
manganite nanoparticles with improved control compared to the traditional ball
milling technique.Comment: 9 pages, 6 figure
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