59 research outputs found
Exciton Fate in Semiconductor Nanocrystals at Elevated Temperatures: Hole Trapping Outcompetes Exciton Deactivation
The tens-of-percent photoluminescence
(PL) quantum yields routinely
obtained for colloidally prepared CdSe semiconductor nanocrystals
(NCs) decrease substantially with temperature elevation. While such
PL efficiency loss has direct consequences for applications ranging
from light-emitting diodes and lasers to photovoltaics under solar
concentration, the origin of this loss is currently not established,
hindering synthetic efforts to design materials with robust performance.
Here, for the first time, we utilize transient absorption and ultrafast
PL in addition to static PL and time-correlated single photon counting,
to characterize CdSe core-only and CdSe/ZnS core/shell NCs up to temperatures
as high as 800 K. For multiple particle sizes, loss of PL efficiency
as a function of temperature elevation is more severe and less reversible
for core-only NCs than for core/shell NCs. Ultrafast measurements
performed at elevated sample temperatures indicate that thermally
activated trapping of individual carriers dominates the nonradiative
loss of excitons. Through a combination of spectroscopic techniques,
we identify the primary carrier loss process as hole trapping in particular.
These findings support the notion that extrinsic trapping effects
out-compete intrinsic exciton deactivation at high temperature and
point to realizable improvements in thermally robust optoelectronic
performance
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 (430–490
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 ×
10<sup>–14</sup> cm<sup>–2</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
Transient Negative Optical Nonlinearity of Indium Oxide Nanorod Arrays in the Full-Visible Range
Dynamic control of
the optical response of materials at visible
wavelengths is key to future metamaterials and photonic integrated
circuits. Materials such as transparent conducting oxides have attracted
significant attention due to their large optical nonlinearity under
resonant optical pumping condition. However, optical nonlinearities
of TCOs are positive in sign and are mostly in the ε-near-zero
to metallic range where materials can become lossy. Here we demonstrate
large amplitude, negative optical nonlinearity (Δ<i>n</i> from −0.05 to −0.09) of indium oxide nanorod arrays
in the full-visible range where the material is transparent. We experimentally
quantify and theoretically calculate the optical nonlinearity, which
arises from a strong modification of interband optical transitions.
The approach toward negative optical nonlinearity can be generalized
to other transparent semiconducting oxides and opens door to reconfigurable,
subwavelength optical components
Carrier Dynamics in Highly Quantum-Confined, Colloidal Indium Antimonide Nanocrystals
Nanometer-sized particles of indium antimonide (InSb) offer opportunities in areas such as solar energy conversion and single photon sources. Here, we measure electron–hole pair dynamics, spectra, and absorption cross sections of strongly quantum-confined colloidal InSb nanocrystal quantum dots using femtosecond transient absorption. For all samples, we observe a bleach feature that develops on ultrafast time scales, which notably moves to lower energy during the first several picoseconds following excitation. We associate this unusual red shift, which becomes larger for larger particles and more distinct at lower sample temperatures, with hot exciton cooling through states that we suggest arise from energetically proximal conduction band levels. From controlled optical excitation intensities, we determine biexciton lifetimes, which range from 2 to 20 ps for the studied 3–6 nm diameter particle sizes
Near-Infrared Photoluminescence Enhancement in Ge/CdS and Ge/ZnS Core/Shell Nanocrystals: Utilizing IV/II–VI Semiconductor Epitaxy
Ge nanocrystals have a large Bohr radius and a small, size-tunable band gap that may engender direct character <i>via</i> strain or doping. Colloidal Ge nanocrystals are particularly interesting in the development of near-infrared materials for applications in bioimaging, telecommunications and energy conversion. Epitaxial growth of a passivating shell is a common strategy employed in the synthesis of highly luminescent II–VI, III–V and IV–VI semiconductor quantum dots. Here, we use relatively unexplored IV/II–VI epitaxy as a way to enhance the photoluminescence and improve the optical stability of colloidal Ge nanocrystals. Selected on the basis of their relatively small lattice mismatch compared with crystalline Ge, we explore the growth of epitaxial CdS and ZnS shells using the successive ion layer adsorption and reaction method. Powder X-ray diffraction and electron microscopy techniques, including energy dispersive X-ray spectroscopy and selected area electron diffraction, clearly show the controllable growth of as many as 20 epitaxial monolayers of CdS atop Ge cores. In contrast, Ge etching and/or replacement by ZnS result in relatively small Ge/ZnS nanocrystals. The presence of an epitaxial II–VI shell greatly enhances the near-infrared photoluminescence and improves the photoluminescence stability of Ge. Ge/II–VI nanocrystals are reproducibly 1–3 orders of magnitude brighter than the brightest Ge cores. Ge/4.9CdS core/shells show the highest photoluminescence quantum yield and longest radiative recombination lifetime. Thiol ligand exchange easily results in near-infrared active, water-soluble Ge/II–VI nanocrystals. We expect this synthetic IV/II–VI epitaxial approach will lead to further studies into the optoelectronic behavior and practical applications of Si and Ge-based nanomaterials
Reverse Non-Equilibrium Molecular Dynamics Demonstrate That Surface Passivation Controls Thermal Transport at Semiconductor–Solvent Interfaces
We examine the role played by surface structure and passivation in thermal transport at semiconductor/organic interfaces. Such interfaces dominate thermal transport in semiconductor nanomaterials owing to material dimensions much smaller than the bulk phonon mean free path. Utilizing reverse nonequilibrium molecular dynamics simulations, we calculate the interfacial thermal conductance (<i>G</i>) between a hexane solvent and chemically passivated wurtzite CdSe surfaces. In particular, we examine the dependence of <i>G</i> on the CdSe slab thickness, the particular exposed crystal facet, and the extent of surface passivation. Our results indicate a nonmonotonic dependence of <i>G</i> on ligand-grafting density, with interfaces generally exhibiting higher thermal conductance for increasing surface coverage up to ∼0.08 ligands/Å<sup>2</sup> (75–100% of a monolayer, depending on the particular exposed facet) and decreasing for still higher coverages. By analyzing orientational ordering and solvent penetration into the ligand layer, we show that a balance of competing effects is responsible for this nonmonotonic dependence. Although the various unpassivated CdSe surfaces exhibit similar <i>G</i> values, the crystal structure of an exposed facet nevertheless plays an important role in determining the interfacial thermal conductance of passivated surfaces, as the density of binding sites on a surface determines the ligand-grafting densities that may ultimately be achieved. We demonstrate that surface passivation can increase <i>G</i> relative to a bare surface by roughly 1 order of magnitude and that, for a given extent of passivation, thermal conductance can vary by up to a factor of ∼2 between different surfaces, suggesting that appropriately tailored nanostructures may direct heat flow in an anisotropic fashion for interface-limited thermal transport
Synthesis and Ligand Exchange of Thiol-Capped Silicon Nanocrystals
Hydride-terminated silicon (Si) nanocrystals
were capped with dodecanethiol
by a thermally promoted thiolation reaction. Under an inert atmosphere,
the thiol-capped nanocrystals exhibit photoluminescence (PL) properties
similar to those of alkene-capped Si nanocrystals, including size-tunable
emission wavelength, relatively high quantum yields (>10%), and
long
radiative lifetimes (26–280 μs). X-ray photoelectron
spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy
confirmed that the ligands attach to the nanocrystal surface via covalent
Si–S bonds. The thiol-capping layer, however, readily undergoes
hydrolysis and severe degradation in the presence of moisture. Dodecanethiol
could be exchanged with dodecene by hydrosilylation for enhanced stability
High-Performance Bioassisted Nanophotocatalyst for Hydrogen Production
Nanophotocatalysis is one of the
potentially efficient ways of
capturing and storing solar energy. Biological energy systems that
are intrinsically nanoscaled can be employed as building blocks for
engineering nanobio-photocatalysts with tunable properties. Here,
we report upon the application of light harvesting proton pump bacteriorhodopsin
(bR) assembled on Pt/TiO<sub>2</sub> nanocatalyst for visible light-driven
hydrogen generation. The hybrid system produces 5275 μmole of
H<sub>2</sub> (μmole protein)<sup>−1</sup> h<sup>–1</sup> at pH 7 in the presence of methanol as a sacrificial electron donor
under white light. Photoelectrochemical and transient absorption studies
indicate efficient charge transfer between bR protein molecules and
TiO<sub>2</sub> nanoparticles
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
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