Perovskite light-emitting diodes (PeLEDs) based on three-dimensional (3D) polycrystalline perovskites suffer from ion migration, which causes overshoot of luminance over time during operation and reduces its operational lifetime. Here, we demonstrate 3D/2D hybrid PeLEDs with extremely reduced luminance overshoot and 21 times longer operational lifetime than 3D PeLEDs. The luminance overshoot ratio of 3D/2D hybrid PeLED is only 7.4% which is greatly lower than that of 3D PeLED (150.4%). The 3D/2D hybrid perovskite is obtained by adding a small amount of neutral benzylamine to methylammonium lead bromide, which induces a proton transfer from methylammonium to benzylamine and enables crystallization of 2D perovskite without destroying the 3D phase. Benzylammonium in the perovskite lattice suppresses formation of deep-trap states and ion migration, thereby enhances both operating stability and luminous efficiency based on its retardation effect in reorientation.
Dimethylammonium
zinc formate ([(CH3)2NH2]Zn(HCOO)3 or DMZnF) is a model system for the
study of hybrid perovskite-like dielectrics. It undergoes a phase
transition from the paraelectric to ferroelectric phase at ∼166
K, as observed via NMR spectra. The mechanism of this phase transition
has been shown to have contributions from ordering of the hydrogen
bonds between [(CH3)2NH2]+ (DMA+) and the formate groups as well as buckling of
the metal-formate framework, but the transition dynamics and atomistic
mechanism are not fully clear. This work presents dielectric constant
measurements as evidence of cluster formation of the low-temperature
phase and the relaxor-like behavior of this metal–organic framework
above the phase transition temperature. 13C CP-MAS is used
to track the evolution of the chemical shift, T
1, and T
2 of the dimethylammonium
cation and formate groups from room temperature to 120 K. 2D 13C–13C correlation measurements provide
evidence of the formation of pretransitional clusters above the phase
transition temperature. Density functional theory (DFT) calculations
support the assignment of chemical shifts and the proposed model.
The analysis of 13C CP-MAS spectra and DFT calculations
is used to discuss the mechanism of the dielectric phase transition
and the origin of relaxor-like behavior in DMZnF.
Densification
in glassy networks has traditionally been described
in terms of short-range structures, such as how atoms are coordinated
and how the coordination polyhedron is linked in the second coordination
environment. While changes in medium-range structures beyond the second
coordination shells may play an important role, experimental verification
of the densification beyond short-range structures is among the remaining
challenges in the physical sciences. Here, a correlation NMR experiment
for prototypical borate glasses under compression up to 9 GPa offers
insights into the pressure-induced evolution of proximity among cations
on a medium-range scale. Whereas amorphous networks at ambient pressure
may favor the formation of medium-range clusters consisting primarily
of similar coordination species, such segregation between distinct
coordination environments tends to decrease with increasing pressure,
promoting a more homogeneous distribution of dissimilar structural
units. Together with an increase in the average coordination number,
densification of glass accompanies a preferential rearrangement toward
a random distribution, which may increase the configurational entropy.
The results highlight the direct link between the pressure-induced
increase in medium-range disorder and the densification of glasses
under extreme compression.
LiH2PO4 (LDP)
is a favored candidate for
hydrogen fuel cells, but the mechanism of its high protonic conductivity
remains unclear. A complicating factor has been the lack of resolution
in the reported proton NMR spectra. We now report multinuclear magic
angle spinning NMR in LDP at magnetic fields up to 21.2 T. Well-resolved 1H NMR spectra are observed that are assignable to protons
in the short and long O–H···O hydrogen bonds
and a peak to physisorbed H2O. The position and intensity
for the H2O peak depend on the H
2
O content, implying fast exchange between the adsorbed H2O and the O–H···O protons. 31P and 7Li NMR spectra and spin–lattice relaxation measurements
showed that the proton hopping/exchange processes involve concerted
hindered rotational fluctuations of the phosphate groups. Conductivity
data from adsorbed H2O-controlled samples clearly suggest
that the mechanism of LDP’s protonic conductivity is dominantly
the exchange (and hopping) of the adsorbed H2O protons
with the short O–H···O hydrogen bonds, in contrast
to an earlier model that ascribed it to intermolecular hopping of
O–H···O protons. The new findings enable us
to modulate LDP’s protonic conductivity by several orders of
magnitude via controlling physisorbed water.
Superprotonic conduction in the LiH2PO4 system has been studied by means of high-resolution nuclear magnetic resonance measurements, which enabled us to distinguish dynamics of the two different hydrogen bonds in the structure. The protonic motion, primarily associated with the longer hydrogen bond, rather than the Li ionic motion, was revealed to dictate the extraordinarily high electrical conductivity of the system.
NH4H2AsO4 (ADA) is a model compound for
understanding the mechanism of phase transitions in the KH2PO4 (KDP) family of ferroelectrics. ADA exhibits a paraelectric
(PE) to antiferroelectric (AFE) phase transition at T
N ∼ 216 K whose mechanism remains unclear. With
the view of probing the role of the various protons in the transition
mechanism, we have employed the high-resolution technique of magic
angle spinning at the high Zeeman field of 21.1 T (1H resonance
at 900 MHz). We measured the temperature dependence of the isotropic
chemical shift and spin–lattice relaxation time, T
1, of the O–H···O and NH4
+ protons through the T
N.
As T → T
N, NMR
peaks from the PE and AFE phases are seen to coexist over a temperature
range of about 3 K, showing formation of nearly static (lifetime >
milliseconds) pretransitional clusters in this lattice as it approaches
its T
N, consistent with the near first-order
nature of the phase transition. The isotropic chemical shift of the
O–H···O protons exhibited a steplike anomaly
at T
N, providing direct evidence of displacive
character in this lattice commonly thought of as an order–disorder
type. No such anomaly was noticeable for the NH4
+ protons. Both sets of protons exhibited order–disorder characteristics
in their T
1 data, as analyzed in terms
of the standard Bloembergen, Purcell, and Pound (BPP) model. These
data suggest that the traditionally employed classification of equilibrium
phase transitions into order–disorder and displacive ones, should rather be “order–disorder
cum displacive” type.
We have studied the metal–insulator transition (MIT) taking place at 340 K in vanadium dioxide nanoparticles. A peculiar nanosize effect on the 10-nm-sized VO2 nanoparticles is reported. While the infrared transmittance at high wave numbers displayed a broad transition behavior, the magnetic susceptibility and infrared transmittance at low wave numbers showed a sharp first-order MIT. Our results suggest that the size effect on the MIT is due to the surface region while the core region undergoes the same MIT as that in the bulk.
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