We studied the phase transition behavior of cubic BaZrO3 perovskite by in situ high pressure synchrotron X-ray diffraction experiments up to 46.4 GPa at room temperature. The phase transition from cubic phase to tetragonal phase was observed in BaZrO3 for the first time, which takes place at 17.2 GPa. A bulk modulus 189 (26) GPa for cubic BaZrO3 is derived from the pressure–volume data. Upon decompression, the high pressure phase transforms into the initial cubic phase. It is suggested that the unstable phonon mode caused by the rotation of oxygen octahedra plays a crucial role in the high pressure phase transition behavior of BaZrO3.
Extending photoelectric response
to the near-infrared (NIR) region
using upconversion luminescent (UCL) materials is one promising approach
to obtain high-efficiency perovskite solar cells (PSCs). However,
challenges remain due to the shortage of highly efficient UCL materials
and device structure. NaCsWO3 nanocrystals exhibit near-infrared
absorption arising from the local surface plasmon resonance (LSPR)
effect, which can be used to boost the UCL of rare-earth-doped upconversion
nanoparticles (UCNPs). In this study, using NaCsWO3 as
the LSPR center, NaCsWO3@NaYF4@NaYF4:Yb,Er nanoparticles were synthesized and the UCL intensity could
be enhanced by more than 124 times when the amount of NaCsWO3 was 2.8 mmol %. Then, such efficient UCNPs were not only doped into
the hole transport layer but also used to modify the perovskite film
in PSCs, resulting in the highest power conversion efficiency (PCE)
reaching 18.89% (that of the control device was 16.01% and the PCE
improvement was 17.99%). Possible factors for the improvement of PSCs
were studied and analyzed. It is found that UCNPs can broaden the
response range of PSCs to the NIR region due to the LSPR-enhanced
UCL and increase the visible light reabsorption of PSCs due to the
scattering and reflection effect, which generate more photocurrent
in PSCs. In addition, UCNPs modify the perovskite film by effectively
filling the holes and gaps at the grain boundary and eliminating the
perovskite surface defects, which lead to less carrier recombination
and then effectively improve the performance of PSC devices.
Transition metal
dichalcogenides (TMDs) have recently gained tremendous interest for
use in electronic and optoelectronic applications. Unfortunately,
the electronic structure or band gap of most TMDs shows noncontinuously
tunable characteristics, which limits their application to energy-variable
optoelectronics. Thus, layered materials with better tunability in
their electronic structures and band gaps are desired. Herein, we
experimentally demonstrated that layered WSe2 possessed
highly tunable transport properties under various pressures, with
a linearly decreasing band gap that culminates in metallization. Pressure
tuned the band gap of WSe2 linearly, at a rate of 25 meV/GPa.
The high tunability of WSe2 was attributed to the larger
electron orbitals of W2+ and Se2– in
WSe2 compared to the Mo2+ and S2– in MoS2. WSe2 underwent an isostructural phase
transition from a 2D layered structure to a 3D structure at approximately
51.7 GPa, where a conversion from van der Waals (vdW) to covalent-like
bonding was observed in the valence electron localization function
(ELF). Our results present an important advance toward controlling
the band structure of layered materials and suggest significant implications
for energy-variable optoelectronic devices via pressure engineering.
Pressure-induced metallization and amorphization in VO 2 (A) nanorods. Physics, 93(18) nanorods is demonstrated, which provides important physical foundation in experimental understanding of MIT in VO 2 . The observed tetragonal metallic state at ∼28 GPa should be interpreted as a distinct metastable state, while increasing pressure to ∼32 GPa, it transforms into a metallic amorphous state completely. The metallization is due to V 3d orbital electrons delocalization, and the amorphization is attributed to the unique variation of V-O-V bond angle. A metallic amorphous VO 2 state is found under pressure, which is beneficial to explore the phase diagram of VO 2 . Furthermore, this work proves the occurrence of both the metallization and amorphization in octahedrally coordinated materials.
Physical Review B. Condensed Matter and Materials
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