8 research outputs found
Biexciton Emission as a Probe of Auger Recombination in Individual Silicon Nanocrystals
Biexciton
emission from individual silicon nanocrystals was detected
at room temperature by time-resolved, single-particle luminescence
measurements. The efficiency of this process, however, was found to
be very low, about 10–20 times less than the single exciton
emission efficiency. It decreases even further at low temperature,
explaining the lack of biexciton emission line observations in silicon
nanocrystal single-dot spectroscopy under high excitation. The poor
efficiency of the biexciton emission is attributed to the dominant
nonradiative Auger process. Corresponding measured biexciton decay
times then represent Auger lifetimes, and the values obtained here,
from tens to hundreds of nanoseconds, reveal strong dot-to-dot variations,
while the range compares well with recent calculations taking into
account the resonant nature of the Auger process in semiconductor
nanocrystals
Rapid Trapping as the Origin of Nonradiative Recombination in Semiconductor Nanocrystals
We demonstrate that nonradiative
recombination in semiconductor
nanocrystals can be described by a rapid luminescence intermittency,
based on carrier tunneling to resonant traps. Such process, we call
it “rapid trapping (blinking)”, leads to delayed luminescence
and promotes Auger recombination, thus lowering the quantum efficiency.
To prove our model, we probed oxide- (containing static traps) and
ligand- (trap-free) passivated silicon nanocrystals emitting at similar
energies and featuring monoexponential blinking statistics. This allowed
us to find analytical formulas and to extract characteristic trapping/detrapping
rates, and quantum efficiency as a function of temperature and excitation
power. Experimental single-dot temperature-dependent decays, supporting
the presence of one or few resonant static traps, and ensemble saturation
curves were found to be very well described by this effect. The model
can be generalized to other semiconductor nanocrystals, although the
exact interplay of trapping/detrapping, radiative, and Auger processes
may be different, considering the typical times of the processes involved
Near-Unity Internal Quantum Efficiency of Luminescent Silicon Nanocrystals with Ligand Passivation
Spectrally resolved photoluminescence (PL) decays were measured for samples of colloidal, ligand-passivated silicon nanocrystals. These samples have PL emission energies with peak positions in the range ∼1.4–1.8 eV and quantum yields of ∼30–70%. Their ensemble PL decays are characterized by a stretched-exponential decay with a dispersion factor of ∼0.8, which changes to an almost monoexponential character at fixed detection energies. The dispersion factors and decay rates for various detection energies were extracted from spectrally resolved curves using a mathematical approach that excluded the effect of homogeneous line width broadening. Since nonradiative recombination would introduce a random lifetime variation, leading to a stretched-exponential decay for an ensemble, we conclude that the observed monoexponential decay in size-selected ensembles signifies negligible nonradiative transitions of a similar strength to the radiative one. This conjecture is further supported as extracted decay rates agree with radiative rates reported in the literature, suggesting 100% internal quantum efficiency over a broad range of emission wavelengths. The apparent differences in the quantum yields can then be explained by a varying fraction of “dark” or blinking nanocrystals
Thermophoresis-Controlled Size-Dependent DNA Translocation through an Array of Nanopores
Large
arrays of nanopores can be used for high-throughput biomolecule
translocation with applications toward size discrimination and sorting
at the single-molecule level. In this paper, we propose to discriminate
DNA length by the capture rate of the molecules to an array of relatively
large nanopores (50–130 nm) by introducing a thermal gradient
by laser illumination in front of the pores balancing the force from
an external electric field. Nanopore arrays defined by photolithography
were batch processed using standard silicon technology in combination
with electrochemical etching. Parallel translocation of single, fluorophore-labeled
dsDNA strands is recorded by imaging the array with a fast CMOS camera.
The experimental data show that the capture rates of DNA molecules
decrease with increasing DNA length due to the thermophoretic effect
of the molecules. It is shown that the translocation can be completely
turned off for the longer molecule using an appropriate bias, thus
allowing a size discrimination of the DNA translocation through the
nanopores. A derived analytical model correctly predicts the observed
capture rate. Our results demonstrate that by combining a thermal
and a potential gradient at the nanopores, such large nanopore arrays
can potentially be used as a low-cost, high-throughput platform for
molecule sensing and sorting
Thermophoresis-Controlled Size-Dependent DNA Translocation through an Array of Nanopores
Large
arrays of nanopores can be used for high-throughput biomolecule
translocation with applications toward size discrimination and sorting
at the single-molecule level. In this paper, we propose to discriminate
DNA length by the capture rate of the molecules to an array of relatively
large nanopores (50–130 nm) by introducing a thermal gradient
by laser illumination in front of the pores balancing the force from
an external electric field. Nanopore arrays defined by photolithography
were batch processed using standard silicon technology in combination
with electrochemical etching. Parallel translocation of single, fluorophore-labeled
dsDNA strands is recorded by imaging the array with a fast CMOS camera.
The experimental data show that the capture rates of DNA molecules
decrease with increasing DNA length due to the thermophoretic effect
of the molecules. It is shown that the translocation can be completely
turned off for the longer molecule using an appropriate bias, thus
allowing a size discrimination of the DNA translocation through the
nanopores. A derived analytical model correctly predicts the observed
capture rate. Our results demonstrate that by combining a thermal
and a potential gradient at the nanopores, such large nanopore arrays
can potentially be used as a low-cost, high-throughput platform for
molecule sensing and sorting
Photostable Polymer/Si Nanocrystal Bulk Hybrids with Tunable Photoluminescence
Solid
polymer/Si nanocrystal bulk nanocomposites were fabricated
from solutions of alkene- and hydride-terminated silicon nanocrystals
(NCs) in toluene. The photoluminescence peak position of hydride-terminated
SiNCs before polymerization was tuned by photoassisted hydrofluoric
acid etching. Optical properties of obtained PMMA/NC hybrids, such
as quantum yield, luminescence lifetime, and dispersion factor, were
evaluated over time. Photostability of these transparent bulk polymer/SiNC
hybrids over months was confirmed. The emission covers the visible
to near-infrared range with a quantum yield of ∼30–40%
for yellow-red nanocomposites
Light-Converting Polymer/Si Nanocrystal Composites with Stable 60–70% Quantum Efficiency and Their Glass Laminates
Thiol–ene
polymer/Si nanocrystal bulk hybrids were synthesized from alkyl-passivated
Si nanocrystal (Si NC) toluene solutions. Radicals in the polymer
provided a copassivation of “dark” Si NCs, making them
optically active and leading to a substantial ensemble quantum yield
increase. Optical stability over several months was confirmed. The
presented materials exhibit the highest photoluminescence quantum
yield (∼65%) of any solid-state Si NC hybrid reported to date.
The broad tunability of thiol–ene polymer reactivity provides
facile glass integration, as demonstrated by a laminated structure.
This, together with extremely fast polymerization, makes the demonstrated
hybrid material a promising candidate for light converting applications
Evolution of the Ultrafast Photoluminescence of Colloidal Silicon Nanocrystals with Changing Surface Chemistry
The
role of surface species in the optical properties of silicon nanocrystals
(SiNCs) is the subject of intense debate. Changes in photoluminescence
(PL) energy following hydrosilylation of SiNCs with alkyl-terminated
surfaces are most often ascribed to enhanced quantum confinement in
the smaller cores of oxidized NCs or to oxygen-induced defect emission.
We have investigated the PL properties of alkyl-functionalized SiNCs
prepared using two related methods: thermal and photochemical hydrosilylation.
Photochemically functionalized SiNCs exhibit higher emission energies
than the thermally functionalized equivalent. While microsecond lifetime
emission attributed to carrier recombination within the NC core was
observed from all samples, much faster, size-independent nanosecond
lifetime components were only observed in samples prepared using photochemical
hydrosilylation that possessed substantial surface oxidation. In addition,
photochemically modified SiNCs exhibit higher absolute photoluminescent
quantum yields (AQY), consistent with radiative recombination processes
occurring at the oxygen-based defects. Correlating spectrally- and
time-resolved PL measurements and XPS-derived relative surface oxidation
for NCs prepared using different photoassisted hydrosilylation reaction
times provides evidence the PL blue-shift as well as the short-lived
PL emission observed for photochemically functionalized SiNCs are
related to the relative concentration of oxygen surface defects