Reducing the dimensionality of three-dimensional hybrid metal halide perovskites can improve their optoelectronic properties. Here, we show that the third-order optical nonlinearity, n
2, of hybrid lead iodide perovskites is enhanced in the two-dimensional Ruddlesden-Popper series, (CH3(CH2)3NH3)2(CH3NH3)n-1PbnI3n+1 (n = 1–4), where the layer number (n) is engineered for bandgap tuning from E
g = 1.60 eV (n = ∞; bulk) to 2.40 eV (n = 1). Despite the unfavorable relation, , strong quantum confinement causes these two-dimensional perovskites to exhibit four times stronger third harmonic generation at mid-infrared when compared with the three-dimensional counterpart, (CH3NH3)PbI3. Surprisingly, however, the impact of dimensional reduction on two-photon absorption, which is the Kramers-Kronig conjugate of n
2, is rather insignificant as demonstrated by broadband two-photon spectroscopy. The concomitant increase of bandgap and optical nonlinearity is truly remarkable in these novel perovskites, where the former increases the laser-induced damage threshold for high-power nonlinear optical applications.
Above-room-temperature polar magnets are of interest due to their practical applications in spintronics. Here we present a strategy to design high-temperature polar magnetic oxides in the corundum-derived A2BB'O6 family, exemplified by the non-centrosymmetric (R3) Ni3TeO6-type Mn(2+)2Fe(3+)Mo(5+)O6, which shows strong ferrimagnetic ordering with TC = 337 K and demonstrates structural polarization without any ions with (n-1)d(10)ns(0), d(0), or stereoactive lone-pair electrons. Density functional theory calculations confirm the experimental results and suggest that the energy of the magnetically ordered structure, based on the Ni3TeO6 prototype, is significantly lower than that of any related structure, and accounts for the spontaneous polarization (68 μC cm(-2)) and non-centrosymmetry confirmed directly by second harmonic generation. These results motivate new directions in the search for practical magnetoelectric/multiferroic materials.
Hybrid organic−inorganic lead halide perovskites have recorded unprecedented improvement in efficiency as fourth-generation photovoltaic materials. Recently, they have attracted enormous interest in nonlinear optics stemming basically from their excellent optoelectronic properties. Here, we investigate multiphoton absorption (MPA) in high-quality MAPbX 3 (MA = CH 3 NH 3 and X = Cl, Br, I) bulk single crystals synthesized by an inversetemperature crystallization (ITC) method. The two-photon absorption (2PA) coefficients under picosecond pulse excitation are determined to be β = 23 ± 2 cm/ GW and 9 ± 1 cm/GW for MAPbI 3 and MAPbBr 3 at λ = 1064 nm, and 13 ± 2 cm/ GW for MAPbCl 3 at λ = 532 nm. The 2PA coefficients are comparable to those of conventional semiconductors having similar bandgaps and can be explained by a two-band model. Furthermore, we characterize the three-photon absorption behavior of MAPbCl 3 at λ = 1064 nm, yielding γ = 0.05 ± 0.01 cm 3 /GW 2 . The polarization dependence of MPA is also probed to experimentally estimate the degree of anisotropy. The hybrid perovskites are promising materials for nonlinear optical applications due to polarizationdependent MPA response and subsequent strong photoluminescence emission, especially for the Br-and I-containing compounds.
The pressure-induced
structural evolution of formamidinium-based
perovskite FAPbI3 was investigated using in situ synchrotron X-ray diffraction and laser-excited photoluminescence
methods. Cubic α-FAPbI3 (Pm3̅m) partially and irreversibly transformed to hexagonal δ-FAPbI3 (P63
mc) at a
pressure less than 0.1 GPa. Structural transitions of α-FAPbI3 followed the sequence of Pm3̅m → P4/mbm → Im3̅ → partial amorphous during compression
to 6.59 GPa, whereas the δ-phase converted to an orthorhombic Cmc21 structure between 1.26 and 1.73 GPa. During
decompression, FAPbI3 recovered the P63
mc structure of the δ-phase as a minor
component (∼18 wt %) from 2.41–1.40 GPa and the Pm3̅m structure of the α-phase
becomes dominant (∼82 wt %) at 0.10 GPa but with an increased
fraction of δ-FAPbI3. The photoluminescence behaviors
from both the α- and δ-forms were likely controlled by
radiative recombination at the defect levels rather than band-edge
emission during pressure cycling. FAPbI3 polymorphism is
exquisitely sensitive to pressure. While modest pressures can engineer
FAPbI3-based photovoltaic devices, irreversible δ-phase
crystallization may be a limiting factor and should be taken into
account.
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