  Although the structural phase transitions in single-crystal hybrid
methyl-ammonium (MA) lead halide perovskites (MAPbX3, X = Cl, Br, I) are common
phenomena, they have never been observed in the corresponding nanocrystals.
Here we demonstrate that two-photon-excited photoluminescence (PL) spectroscopy
is capable of monitoring the structural phase transitions in MAPbX3
nanocrystals because nonlinear susceptibilities govern the light absorption
rates. We provide experimental evidence that the orthorhombic-to-tetragonal
structural phase transition in a single layer of 20-nm-sized 3D MAPbBr3
nanocrystals is spread out within the 70 - 140 K range. This structural phase
instability range arises because, unlike in single-crystal MAPbX3, free
rotations of MA ions in the corresponding nanocrystals are no longer restricted
by a long-range MA dipole order. The resulting configurational entropy loss can
be even enhanced by the interfacial electric field arising due to charge
separation at the MAPbBr3/ZnO heterointerface, extending the
orthorhombic-to-tetragonal structural phase instability range from 70 to 230 K.
We conclude that the weak sensitivity of conventional one-photon-excited PL
spectroscopy to the structural phase transitions in 3D MAPbX3 nanocrystals
results from the structural phase instability providing negligible distortions
of PbX6 octahedra. In contrast, the intensity of two-photon-excited PL and
electric-field-induced one-photon-excited PL still remains sensitive enough to
weak structural distortions due to the higher rank tensor nature of nonlinear
susceptibilities involved. We also show that room-temperature PL originates
from the radiative recombination of the optical-phonon vibrationally excited
polaronic quasiparticles with energies might exceed the ground-state Frohlich
polaron and Rashba energies due to optical-phonon bottleneck.
