Solution-processable
metal halide perovskites show immense promise
for use in photovoltaics and other optoelectronic applications. The
ability to tune their bandgap by alloying various halide anions (for
example, in CH3NH3Pb(I1–x
Br
x
)3, 0 < x < 1) is however hampered by the reversible photoinduced
formation of sub-bandgap emissive states. We find that ion segregation
takes place via halide defects, resulting in iodide-rich low-bandgap
regions close to the illuminated surface of the film. This segregation
may be driven by the strong gradient in carrier generation rate through
the thickness of these strongly absorbing materials. Once returned
to the dark, entropically driven intermixing of halides returns the
system to a homogeneous condition. We present approaches to suppress
this process by controlling either the internal light distribution
or the defect density within the film. These results are relevant
to stability in both single- and mixed-halide perovskites, leading
the way toward tunable and stable perovskite thin films for photovoltaic
and light-emitting applications.
We report a hafnium-containing MOF, hcp UiO-67(Hf),
which is a ligand-deficient layered analogue of the face-centered
cubic fcu UiO-67(Hf). hcp UiO-67 accommodates
its lower ligand:metal ratio compared to fcu UiO-67 through
a new structural mechanism: the formation of a condensed “double
cluster” (Hf12O8(OH)14), analogous
to the condensation of coordination polyhedra in oxide frameworks.
In oxide frameworks, variable stoichiometry can lead to more complex
defect structures, e.g., crystallographic shear planes or modules
with differing compositions, which can be the source of further chemical
reactivity; likewise, the layered hcp UiO-67 can react
further to reversibly form a two-dimensional metal–organic
framework, hxl UiO-67. Both three-dimensional hcp UiO-67 and two-dimensional hxl UiO-67 can be delaminated
to form metal–organic nanosheets. Delamination of hcp UiO-67 occurs through the cleavage of strong hafnium-carboxylate
bonds and is effected under mild conditions, suggesting that defect-ordered
MOFs could be a productive route to porous two-dimensional materials.
Multi-temperature X-ray diffraction studies show that twisting, rotation, and libration cause negative thermal expansion (NTE) of the nanoporous metal−organic framework MOF-5, Zn4O(1,4-benzenedicarboxylate)3. The near-linear lattice contraction is quantified in the temperature range 80−500 K using synchrotron powder X-ray diffraction. Vibrational motions causing the abnormal expansion behavior are evidenced by shortening of certain interatomic distances with increasing temperature according to single-crystal X-ray diffraction on a guest-free crystal over a broad temperature range. Detailed analysis of the atomic positional and displacement parameters suggests two contributions to cause the effect: (1) local twisting and vibrational motion of the carboxylate groups and (2) concerted transverse vibration of the linear linkers. The vibrational mechanism is confirmed by calculations of the dynamics in a molecular fragment of the framework.
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