2018 New's Items

Photoexcited Small Polaron Formation in Goethite (α-FeOOH) Nanorods Probed by Transient Extreme Ultraviolet Spectroscopy

Ilana J. Porter
Scott K. Cushing
Lucas M. Carneiro
Angela Lee
Justin C. Ondry
Jakob C. Dahl
Hung-Tzu Chang
A. Paul Alivisatos
Stephen R. Leone
2018

Small polaron formation limits the mobility and lifetimes of photoexcited carriers in metal oxides. As the ligand field strength increases, the carrier mobility decreases, but the effect on the photoexcited small polaron formation is still unknown. Extreme ultraviolet transient absorption spectroscopy is employed to measure small polaron formation rates and probabilities in goethite (α-FeOOH) crystalline nanorods at pump photon energies from 2.2 to 3.1 eV. The measured polaron formation time increases with excitation photon energy from 70 ± 10 fs at 2.2 eV to 350 ± 30 fs at 2.6 eV,...

Single-Crystal X-ray Diffraction Structures of Covalent Organic Frameworks

T. Ma
E. A. Kapustin
S. X. Yin
L. Liang
Z. Zhou
J. Niu
L. Li
Y. Wang
J. Su
J. Li
X. Wang
W. D. Wang
W. Wang
J. Sun
O. M. Yaghi
2018

The crystallization problem is an outstanding challenge in the chemistry of porous covalent organic frameworks (COFs). Their structural characterization has been limited to modeling and solutions based on powder x-ray or electron diffraction data. Single crystals of COFs amenable to x-ray diffraction characterization have not been reported. Here, we developed a general procedure to grow large single crystals of three-dimensional imine-based COFs (COF-300, hydrated form of COF-300, COF-303, LZU-79, and LZU-111). The high quality of the crystals allowed collection of single-crystal x-...

Tunable Polaron Distortions Control the Extent of Halide Demixing in Lead Halide Perovskites

C.G. Bischak
A.B. Wong
E. Lin
D.T. Limmer
P. Yang
N. S. Ginsberg
2018

Photoinduced phase separation in mixed halide perovskites emerges from their electro-mechanical properties and high ionic conductivities, resulting in photoinduced I–-rich charge carrier traps that diminish photovoltaic performance. Whether photoinduced phase separation stems from the polycrystalline microstructure or is an intrinsic material property has been an open question. We investigate the nanoscale photoinduced behavior of single-crystal mixed Br–/I– methylammonium (MA+)...

Cooperative Gas Adsorption without a Phase Transition in Metal-Organic Frameworks

J. Kundu
J. F. Stilck
J.-H. Lee
J. B. Neaton
D. Prendergast
S. Whitelam
2018

Cooperative adsorption of gases by porous frameworks, which permits more efficient uptake and removal than the more usual noncooperative (Langmuir-type) adsorption, usually results from a phase transition of the framework. Here we show how cooperativity emerges in the class of metal-organic frameworks mmen-M2(dobpdc)...

Excited-state Vibrational Dynamics toward the Polaron in Methylammonium Lead Iodide Perovskite

M. Park
A. J. Neukirch
S. E. Reyes-Lillo
M. Lai
S. R. Ellis
D. Dietze
J. B. Neaton
P. Yang
S. Tretiak
R. A. Mathies
2018

Hybrid organic–inorganic perovskites have attractive optoelectronic properties including exceptional solar cell performance. The improved properties of perovskites have been attributed to polaronic effects involving stabilization of localized charge character by structural deformations and polarizations. Here we examine the Pb–I structural dynamics leading to polaron formation in methylammonium lead iodide perovskite by transient absorption, time-domain Raman spectroscopy, and density functional theory. Methylammonium lead iodide perovskite exhibits excited-state coherent nuclear...

Impact of Disordered Guest-Framework Interactions on the Crystallography of Metal-Organic Frameworks

S. Lee
H. Buergi
S. A. Alshmimri
O. M. Yaghi
2018

It is a general and common practice to carry out single-crystal X-ray diffraction experiments at cryogenic temperatures in order to obtain high-resolution data. In this report, we show that this practice is not always applicable to metal–organic frameworks (MOFs), especially when these structures are highly porous. Specifically, two new MOFs are reported here, MOF-1004 and MOF-1005, for which the collection of the diffraction data at lower temperature (100 K) did not give data of sufficient quality to allow structure solution. However, collection of data at higher temperature (290 K...

Bright, Mechanosensitive Upconversion with Cubic-Phase Heteroepitaxial Core–Shell Nanoparticles

A. Lay
C. Siefe
S. Fischer
R. Mehlenbacher
F. Ke
W. Mao
A. P. Alivisatos
M. Goodman
J. Dionne
2018

Lanthanide-doped nanoparticles are an emerging class of optical sensors, exhibiting sharp emission peaks, high signal-to-noise ratio, photostability, and a ratiometric color response to stress. The same centrosymmetric crystal field environment that allows for high mechanosensitivity in the cubic-phase (α), however, contributes to low upconversion quantum yield (UCQY). In this work, we engineer brighter mechanosensitive upconverters using a core–shell geometry. Sub-25 nm α-NaYF4:Yb,Er cores are shelled with an optically inert surface passivation layer of ∼4....

Tailoring Morphology of Cu–Ag Nanocrescents and Core–Shell Nanocrystals Guided by a Thermodynamic Model

W. Osowiecki
X. Ye
P. Satish
K. Bustillo
E. Clark
A. P. Alivisatos
2018

The ability to predict and control the formation of bimetallic heterogeneous nanocrystals is desirable for many applications in plasmonics and catalysis. Here, we report the synthesis and characterization of stable, monodisperse, and solution-processed Cu–Ag bimetallic nanoparticles with specific but unusual elemental arrangements that are consistent with a recently developed thermodynamic model. Using air-free scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy, the distribution of Cu and Ag positions was unambiguously identified within individual...

Combinatorial Alloying Improves Bismuth Vanadate Photoanodes via Reduced Monoclinic Distortion

P. Newhouse
D. Guevarra
M. Umehara
S. Reyes-Lillo
L. Zhou, D. Boyd
S. Suram
J. Cooper
J. Haber
J. B. Neaton
J. M. Gregoire
2018

Improving the efficiency of solar-powered oxygen evolution is both critical for development of solar fuels technologies and challenging due to the broad set of properties required of a solar fuels photoanode. Bismuth vanadate, in particular the monoclinic clinobisvanite phase, has received substantial attention and has exhibited the highest radiative efficiency among metal oxides with a band gap in the visible range. Efforts to further improve its photoelectrochemical performance have included alloying one or more metals onto the Bi and/or V sites, with progress on this frontier...

Practical Water Production from Desert Air

F. Fathieh
M. J. Kalmutzki
E. A. Kapustin
P. J. Waller
J. Yang
O. M. Yaghi
2018

Energy-efficient production of water from desert air has not been developed. A proof-of-concept device for harvesting water at low relative humidity was reported; however, it used external cooling and was not desert-tested. We report a laboratory-to-desert experiment where a prototype using up to 1.2 kg of metal-organic framework (MOF)–801 was tested in the laboratory and later in the desert of Arizona, USA. It produced 100 g of water per kilogram of MOF-801 per day-and-night cycle, using only natural cooling and ambient sunlight as a source of energy. We also report an aluminum-...