13 research outputs found
Giant Nanocrystal Quantum Dots: Stable Down-Conversion Phosphors that Exploit a Large Stokes Shift and Efficient Shell-to-Core Energy Relaxation
A new class of nanocrystal quantum dot (NQD), the āgiantā
NQD (g-NQD), was investigated for its potential to address outstanding
issues associated with the use of NQDs as down-conversion phosphors
in light-emitting devices, namely, insufficient chemical/photostability
and extensive self-reabsorption when packed in high densities or in
thick films. Here, we demonstrate that g-NQDs afford significantly
enhanced operational stability compared to their conventional NQD
counterparts and minimal self-reabsorption losses. The latter results
from a characteristic large Stokes shift (>100 nm; >0.39 eV),
which
itself is a manifestation of the internal structure of these uniquely
thick-shelled NQDs. In carefully prepared g-NQDs, light absorption
occurs predominantly in the shell but emission occurs exclusively
from the core. We directly compare for the first time the processes
of shellācore energy relaxation and coreācore energy
transfer by evaluating CdSāCdSe down-conversion of blueāred
light in g-NQDs and in a comparable mixed-NQD (CdSe and CdS) thin
film, revealing that the internal energy relaxation process affords
a more efficient and color-pure conversion of blue to red light compared
to energy transfer. Lastly, we demonstrate the facile fabrication
of white-light devices with correlated color temperature tuned from
ā¼3200 to 5800 K
Comprehensive Analysis of the Effects of CdSe Quantum Dot Size, Surface Charge, and Functionalization on Primary Human Lung Cells
The growing potential of quantum dots (QDs) in applications as diverse as biomedicine and energy has provoked much dialogue about their conceivable impact on human health and the environment at large. Consequently, there has been an urgent need to understand their interaction with biological systems. Parameters such as size, composition, surface charge, and functionalization can be modified in ways to either enhance biocompatibility or reduce their deleterious effects. In the current study, we simultaneously compared the impact of size, charge, and functionalization alone or in combination on biological responses using primary normal human bronchial epithelial cells. Using a suite of cellular end points and gene expression analysis, we determined the biological impact of each of these properties. Our results suggest that positively charged QDs are significantly more cytotoxic compared to negative QDs. Furthermore, while QDs functionalized with long ligands were found to be more cytotoxic than those functionalized with short ligands, negative QDs functionalized with long ligands also demonstrated size-dependent cytotoxicity. We conclude that QD-elicited cytotoxicity is not a function of a single property but a combination of factors. The mechanism of toxicity was found to be independent of reactive oxygen species formation, as cellular viability could not be rescued in the presence of the antioxidant <i>n</i>-acetyl cysteine. Further exploring these responses at the molecular level, we found that the relatively benign negative QDs increased gene expression of proinflammatory cytokines and those associated with DNA damage, while the highly toxic positive QDs induced changes in genes associated with mitochondrial function. In an attempt to tentatively ārankā the contribution of each property in the observed QD-induced responses, we concluded that QD charge and ligand length, and to a lesser extent, size, are key factors that should be considered when engineering nanomaterials with minimal bioimpact (charge > functionalization > size)
Multistate Blinking and Scaling of Recombination Rates in Individual Silica-Coated CdSe/CdS Nanocrystals
Nonradiative
Auger recombination is the primary exciton loss mechanism
in colloidal nanocrystals and an impediment for prospective optoelectronic
applications. Recent development of new core/shell nanocrystals with
suppressed Auger recombination rates has opened the possibility for
studying multicarrier states using time-resolved photoluminescence
(PL) spectroscopy. An important aspect not addressed in previous works
is the scaling of radiative and nonradiative decay rates with the
increasing number and type of excitons in individual nanocrystals.
Here we conduct extensive single-dot PL spectroscopy of emissive states
in PL blinking trajectories of giant silica-coated CdSe/CdS nanocrystals.
At low fluences, we observe the appearance of neutral and charged
exciton (trion) states. Both negative and positive trions show strongly
suppressed Auger recombination rates resulting in PL quantum yields
close to 50%. At higher excitation powers, we observe consecutive
emergence of lower efficiency states, indicative of higher order excitons.
We employ a scaling model for Auger and radiative decay rates and
attribute these states to doubly charged excitons, biexcitons, and
a triexciton. Simultaneous analysis of the second-order correlation
statistics proves that the biexciton Auger recombination channel can
be represented in terms of the superposition of independent recombination
channels of trions. Analysis of the PL emission of the triexciton
state suggests nonstatistical scaling, likely due to the involvement
of the transitions between different symmetries. Finally, measurements
at high excitation fluence of nanocrystals with low trion quantum
yields does not reveal any higher order excitonic states, corroborating
the validity of the scaling model and confirming Auger-related mechanisms
responsible for blinking behavior in such core/shell nanocrystals
New Insights into the Complexities of Shell Growth and the Strong Influence of Particle Volume in Nonblinking āGiantā Core/Shell Nanocrystal Quantum Dots
The growth of ultra-thick inorganic CdS shells over CdSe
nanocrystal quantum dot (NQD) cores gives rise to a distinct class
of NQD called the āgiantā NQD (g-NQD). g-NQDs are characterized
by unique photophysical properties compared to their conventional
core/shell NQD counterparts, including suppressed fluorescence intermittency
(blinking), photobleaching, and nonradiative Auger recombination.
Here, we report new insights into the numerous synthetic conditions
that influence the complex process of thick-shell growth. We show
the individual and collective effects of multiple reaction parameters
(noncoordinating solvent and coordinating-ligand identities and concentrations,
precursor/NQD ratios, precursor reaction times, etc.) on determining
g-NQD shape and crystalline phase, and the relationship between these
structural features and optical properties. We find that hexagonally
faceted wurzite g-NQDs afford the highest ensemble quantum yields
in emission and the most complete suppression of blinking. Significantly,
we also reveal a clear correlation between g-NQD particle volume and
blinking suppression, such that larger cores afford blinking-suppressed
behavior at relatively thinner shells compared to smaller starting
core sizes, which require application of thicker shells to realize
the same level of blinking suppression. We show that there is a common,
threshold g-NQD volume (ā¼750 nm<sup>3</sup>) that is required
to observe blinking suppression and that this particle volume corresponds
to an NQD radiative lifetime of ā¼65 ns regardless of starting
core size. Combining new understanding of key synthetic parameters
with optimized core/shell particle volumes, we demonstrate effectively
complete suppression of blinking even for long observation times of
ā¼1 h
āGiantā CdSe/CdS Core/Shell Nanocrystal Quantum Dots As Efficient Electroluminescent Materials: Strong Influence of Shell Thickness on Light-Emitting Diode Performance
We use a simple device architecture based on a polyĀ(3,4-ethylendioxythiophene):polyĀ(styrenesulfonate)
(PEDOT:PSS)-coated indium tin oxide anode and a LiF/Al cathode to
assess the effects of shell thickness on the properties of light-emitting
diodes (LEDs) comprising CdSe/CdS core/shell nanocrystal quantum dots
(NQDs) as the emitting layer. Specifically, we are interested in determining
whether LEDs based on thick-shell nanocrystals, so-called āgiantā
NQDs, afford enhanced performance compared to their counterparts incorporating
thin-shell systems. We observe significant improvements in device
performance as a function of increasing shell thickness. While the
turn-on voltage remains approximately constant for all shell thicknesses
(from 4 to 16 CdS monolayers), external quantum efficiency and maximum
luminance are found to be about one order of magnitude higher for
thicker shell nanocrystals (ā„13 CdS monolayers) compared to
thinner shell structures (<9 CdS monolayers). The thickest-shell
nanocrystals (16 monolayers of CdS) afforded an external quantum efficiency
and luminance of 0.17% and 2000 Cd/m<sup>2</sup>, respectively, with
a remarkably low turn-on voltage of ā¼3.0 V
Single-Nanocrystal Photoluminescence Spectroscopy Studies of PlasmonāMultiexciton Interactions at Low Temperature
Using thick-shell or āgiantā
CdSe/CdS nanocrystal
quantum dots (g-NQDs), characterized by strongly suppressed Auger
recombination, we studied the influence of plasmonic interactions
on multiexciton emission. Specifically, we assessed the separate effects
of plasmonic absorption and plasmonic emission enhancement by a systematic
analysis of the pump fluence dependence of low-temperature photoluminescence
(low-<i>T</i> PL) derived from individual CdSe/CdS g-NQDs
deposited on nanoroughened silver films. Our study reveals that (1)
the multiexciton (MX) emissions in g-NQD coupled to silver films were
enhanced not only through the creation of more excitons via enhancement
of absorption but also through the direct modification of the competition
between the radiative and nonradiative recombination processes of
MXs; (2) strong enhancement in absorption is not necessary for strong
multiexciton emission; and (3) the emission of MXs can become stronger
with the increase of multiexciton order. We also exploited the strong
enhancement of MX emission to perform second-order photon correlation
and cross-correlation experiments using very low pump fluences and
observed a strong photon bunching that decays with increasing pump
fluence
Elucidation of Two Giants: Challenges to Thick-Shell Synthesis in CdSe/ZnSe and ZnSe/CdS Core/Shell Quantum Dots
Core/thick-shell
giant quantum dots (gQDs) possessing type II electronic
structures exhibit suppressed blinking and diminished nonradiative
Auger recombination. We investigate CdSe/ZnSe and ZnSe/CdS as potential
new gQDs. We show theoretically and experimentally that both can exhibit
partial or complete spatial separation of an excited-state electronāhole
pair (i.e., type II behavior). However, we reveal that thick-shell
growth is challenged by competing processes: alloying and cation exchange.
We demonstrate that these can be largely avoided by choice of shelling
conditions (e.g., time, temperature, and QD core identity). The resulting
CdSe/ZnSe gQDs exhibit unusual single-QD properties, principally emitting
from dim gray states but having high two-exciton (biexciton) emission
efficiencies, whereas ZnSe/CdS gQDs show characteristic gQD blinking
suppression, though only if shelling is accompanied by partial cation
exchange
Matching Solid-State to Solution-Phase Photoluminescence for Near-Unity Down-Conversion Efficiency Using Giant Quantum Dots
Efficient,
stable, and narrowband red-emitting fluorophores are
needed as down-conversion materials for next-generation solid-state
lighting that is both efficient and of high color quality. Semiconductor
quantum dots (QDs) are nearly ideal color-shifting phosphors, but
solution-phase efficiencies have not traditionally extended to the
solid-state, with losses from both intrinsic and environmental effects.
Here, we assess the impacts of temperature and flux on QD phosphor
performance. By controlling QD core/shell structure, we realize near-unity
down-conversion efficiency and enhanced operational stability. Furthermore,
we show that a simple modification of the phosphor-coated light-emitting
diode deviceāincorporation of a thin spacer layerācan
afford reduced thermal or photon-flux quenching at high driving currents
(>200 mA)
Quantum Yield Heterogeneity among Single Nonblinking Quantum Dots Revealed by Atomic Structure-Quantum Optics Correlation
Physical variations in colloidal
nanostructures give rise to heterogeneity
in expressed optical behavior. This correlation between nanoscale
structure and function demands interrogation of both atomic structure
and photophysics at the level of single nanostructures to be fully
understood. Herein, by conducting detailed analyses of fine atomic
structure, chemical composition, and time-resolved single-photon photoluminescence
data for the same individual nanocrystals, we reveal inhomogeneity
in the quantum yields of single nonblinking āgiantā
CdSe/CdS core/shell quantum dots (g-QDs). We find that each g-QD possesses
distinctive single exciton and biexciton quantum yields that result
mainly from variations in the degree of charging, rather than from
volume or structure inhomogeneity. We further establish that there
is a very limited nonemissive ādarkā fraction (<2%)
among the studied g-QDs and present direct evidence that the g-QD
core must lack inorganic passivation for the g-QD to be ādarkā.
Therefore, in contrast to conventional QDs, ensemble photoluminescence
quantum yield is principally defined by charging processes rather
than the existence of dark g-QDs
Functionalization-Dependent Induction of Cellular Survival Pathways by CdSe Quantum Dots in Primary Normal Human Bronchial Epithelial Cells
Quantum dots (QDs) are semiconductor nanocrystals exhibiting unique optical properties that can be exploited for many practical applications ranging from photovoltaics to biomedical imaging and drug delivery. A significant number of studies have alluded to the cytotoxic potential of these materials, implicating Cd-leaching as the causal factor. Here, we investigated the role of heavy metals in biological responses and the potential of CdSe-induced genotoxicity. Our results indicate that, while negatively charged QDs are relatively noncytotoxic compared to positively charged QDs, the same does not hold true for their genotoxic potential. Keeping QD core composition and size constant, 3 nm CdSe QD cores were functionalized with mercaptopropionic acid (MPA) or cysteamine (CYST), resulting in negatively or positively charged surfaces, respectively. CYST-QDs were found to induce significant cytotoxicity accompanied by DNA strand breakage. However, MPA-QDs, even in the absence of cytotoxicity and reactive oxygen species formation, also induced a high number of DNA strand breaks. QD-induced DNA damage was confirmed by identifying the presence of p53 binding protein 1 (53BP1) in the nuclei of exposed cells and subsequent diminishment of p53 from cytoplasmic cellular extracts. Further, high-throughput real-time PCR analyses revealed upregulation of DNA damage and response genes and several proinflammatory cytokine genes. Most importantly, transcriptome sequencing revealed upregulation of the metallothionein family of genes in cells exposed to MPA-QDs but not CYST-QDs. These data indicate that cytotoxic assays must be supplemented with genotoxic analyses to better understand cellular responses and the full impact of nanoparticle exposure when making recommendations with regard to risk assessment