57 research outputs found
Heating and cooling of ligand-coated colloidal nanocrystals in solid films and solvent matrices
Unique Optical Properties of Methylammonium Lead Iodide Nanocrystals Below the Bulk Tetragonal-Orthorhombic Phase Transition
Methylammonium
(MA) and formamidinium (FA) lead halides are widely
studied for their potential as low-cost, high-performance optoelectronic
materials. Here, we present measurements of visible and IR absorption,
steady state, and time-resolved photoluminescence from 300 K to cryogenic
temperatures. Whereas FAPbI<sub>3</sub> nanocrystals (NCs) are found
to behave in a very similar manner to reported bulk behavior, colloidal
nanocrystals of MAPbI<sub>3</sub> show a departure from the low-temperature
optical behavior of the bulk material. Using photoluminescence, visible,
and infrared absorption measurements, we demonstrate that unlike single
crystals and polycrystalline films NCs of MAPbI<sub>3</sub> do not
undergo optical changes associated with the bulk tetragonal-to-orthorhombic
phase transition, which occurs near 160 K. We find no evidence of
frozen organic cation rotation to as low as 80 K or altered exciton
binding energy to as low as 3 K in MAPbI<sub>3</sub> NCs. Similar
results are obtained in MAPbI<sub>3</sub> NCs ranging from 20 to over
100 nm and in morphologies including cubes and plates. Colloidal MAPbI<sub>3</sub> NCs therefore offer a window into the properties of the solar-relevant,
room-temperature phase of MAPbI<sub>3</sub> at temperatures inaccessible
with single crystals or polycrystalline samples. Exploiting this phenomenon,
these measurements reveal the existence of an optically passive photoexcited
state close to the band edge and persistent slow Auger recombination
at low temperature
Violet-to-Blue Gain and Lasing from Colloidal CdS Nanoplatelets: Low-Threshold Stimulated Emission Despite Low Photoluminescence Quantum Yield
Amplified spontaneous emission (ASE)
and lasing from solution-processed
materials are demonstrated in the challenging violet-to-blue (430â490
nm) spectral region for colloidal nanoplatelets of CdS and newly synthesized
core/shell CdS/ZnS nanoplatelets. Despite modest band-edge photoluminescence
quantum yields of 2% or less for single excitons, which we show results
from hole trapping, the samples exhibit low ASE thresholds. Furthermore,
four-monolayer CdS samples show ASE at shorter wavelengths than any
reported film of colloidal quantum-confined material. This work underlines
that low quantum yields for single excitons do not necessarily lead
to a poor gain medium. The low ASE thresholds originate from negligible
dispersion in thickness, large absorption cross sections of 2.8 Ă
10<sup>â14</sup> cm<sup>â2</sup>, and rather slow (150
to 300 ps) biexciton recombination. We show that under higher-fluence
excitation, ASE can kinetically outcompete hole trapping. Using nanoplatelets
as the gain medium, lasing is observed in a linear optical cavity.
This work confirms the fundamental advantages of colloidal quantum
well structures as gain media, even in the absence of high photoluminescence
efficiency
Large Transient Optical Modulation of Epsilon-Near-Zero Colloidal Nanocrystals
Epsilon-near-zero
materials may be synthesized as colloidal nanocrystals
which display large magnitude subpicosecond switching of infrared
localized surface plasmon resonances. Such nanocrystals offer a solution-processable,
scalable source of tunable metamaterials compatible with arbitrary
substrates. Under intraband excitation, these nanocrystals display
a red-shift of the plasmon feature arising from the low electron heat
capacities and conduction band nonparabolicity of the oxide. Under
interband pumping, they show in an ultrafast blueshift of the plasmon
resonance due to transient increases in the carrier density. Combined
with their high-quality factor, large changes in relative transmittance
(+86%) and index of refraction (+85%) at modest control fluences (<5
mJ/cm<sup>2</sup>) suggest that these materials offer great promise
for all-optical switching, wavefront engineering, and beam steering
operating at terahertz switching frequencies
Facile, Economic and Size-Tunable Synthesis of Metal Arsenide Nanocrystals
Synthesis of colloidal
nanocrystals (NC) of important arsenide
nanomaterials (e.g., InAs, Cd<sub>3</sub>As<sub>2</sub>) has been
limited by the lack of convenient arsenic precursors. Here we address
this constraint by identifying a convenient and commercially available
As precursor, tris-dimethylaminoarsine (AsÂ(NMe<sub>2</sub>)<sub>3</sub>), which can be used to prepare high quality InAs NCs with controlled
size distributions. Our approach employs a reaction between InCl<sub>3</sub> and AsÂ(NMe<sub>2</sub>)<sub>3</sub> using diisobutylaluminum
hydride (DIBAL-H) to convert AsÂ(NMe<sub>2</sub>)<sub>3</sub> in situ
into reactive intermediates AsH<sub><i>x</i></sub>(NMe<sub>2</sub>)<sub>3â<i>x</i></sub>, where <i>x</i> = 1,2,3. NC size can be varied by changing DIBAL-H concentration
and growth temperature, with colloidal solutions of InAs showing size
dependent absorption and emission features tunable across wavelengths
of 750 to 1450 nm. We also show that this approach works well for
the colloidal synthesis of Cd<sub>3</sub>As<sub>2</sub> NCs. By circumventing
the preparation of notoriously unstable and dangerous arsenic precursors
(e.g., AsH<sub>3</sub> and AsÂ(SiMe<sub>3</sub>)<sub>3</sub>), this
work improves the synthetic accessibility of arsenide-based NCs and,
by extension, the potential of such NCs for use in infrared (IR) applications
such as communications, fluorescent labeling and photon detection
Surface-Area-Dependent Electron Transfer Between Isoenergetic 2D Quantum Wells and a Molecular Acceptor
We report measurements
of electron transfer rates for four isoenergetic
donorâacceptor pairs comprising a molecular electron acceptor,
methylviologen (MV), and morphology-controlled colloidal semiconductor
nanoparticles of CdSe. The four nanoparticles include a spherical
quantum dot (QD) and three differing lateral areas of 4-monolayer-thick
nanoplatelets (NPLs), each with a 2.42 eV energy gap. As such, the
measurements, performed via ultrafast photoluminescence, relate the
dependence of charge transfer rate on the spatial extent of the initial
electronâhole pair wave function explicitly, which we show
for the first time to be related to surface area in this regime that
is intermediate between homogeneous and heterogeneous charge transfer
as well as 2D to 0D electron transfer. The observed nonlinear dependence
of rate with surface area is attributed to exciton delocalization
within each structure, which we show via temperature-dependent absorption
measurements remains constant
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