12 research outputs found
Room Temperature Single-Photon Emission from Individual Perovskite Quantum Dots
Lead-halide-based perovskites have been the subject of numerous recent studies largely motivated by their exceptional performance in solar cells. Electronic and optical properties of these materials have been commonly controlled by varying the composition (<i>e.g.</i>, the halide component) and/or crystal structure. Use of nanostructured forms of perovskites can provide additional means for tailoring their functionalities <i>via</i> effects of quantum confinement and wave function engineering. Furthermore, it may enable applications that explicitly rely on the quantum nature of electronic excitations. Here, we demonstrate that CsPbX<sub>3</sub> quantum dots (X = I, Br) can serve as room-temperature sources of quantum light, as indicated by strong photon antibunching detected in single-dot photoluminescence measurements. We explain this observation by the presence of fast nonradiative Auger recombination, which renders multiexciton states virtually nonemissive and limits the fraction of photon coincidence events to âŒ6% on average. We analyze limitations of these quantum dots associated with irreversible photodegradation and fluctuations (âblinkingâ) of the photoluminescence intensity. On the basis of emission intensity-lifetime correlations, we assign the âblinkingâ behavior to random charging/discharging of the quantum dot driven by photoassisted ionization. This study suggests that perovskite quantum dots hold significant promise for applications such as quantum emitters; however, to realize this goal, one must resolve the problems of photochemical stability and photocharging. These problems are largely similar to those of more traditional quantum dots and, hopefully, can be successfully resolved using advanced methodologies developed over the years in the field of colloidal nanostructures
Auger Recombination of Biexcitons and Negative and Positive Trions in Individual Quantum Dots
Charged exciton states commonly occur both in spectroscopic studies of quantum dots (QDs) and during operation of QD-based devices. The extra charge added to the neutral exciton modifies its radiative decay rate and also opens an additional nonradiative pathway associated with an Auger process whereby the recombination energy of an exciton is transferred to the excess charge. Here we conduct single-dot spectroscopic studies of Auger recombination in thick-shell (âgiantâ) CdSe/CdS QDs with and without an interfacial alloy layer using time-tagged, time-correlated single-photon counting. In photoluminescence (PL) intensity trajectories of some of the dots, we resolve three distinct states of different emissivities (âbrightâ, âgrayâ, and âdarkâ) attributed, respectively, to the neutral exciton and negative and positive trions. Simultaneously acquired PL lifetime trajectories indicate that the positive trion is much shorter lived than the negative trion, which can be explained by a high density of valence band states and a small hole localization radius (defined by the QD core size), factors that favor an Auger process involving intraband excitation of a hole. A comparison of trion and biexciton lifetimes suggests that the biexciton Auger decay can be treated in terms of a superposition of two independent channels associated with positive- and negative-trion pathways. The resulting interdependence between Auger time constants might simplify the studies of multicarrier recombination by allowing one, for example, to infer Auger lifetimes of trions of one sign based on the measurements of biexciton decay and dynamics of the trions of the opposite sign or, alternatively, estimate the biexciton lifetime based on studies of trion dynamics
Effect of Interfacial Alloying versus âVolume Scalingâ on Auger Recombination in Compositionally Graded Semiconductor Quantum Dots
Auger recombination
is a nonradiative three-particle process wherein
the electronâhole recombination energy dissipates as a kinetic
energy of a third carrier. Auger decay is enhanced in quantum-dot
(QD) forms of semiconductor materials compared to their bulk counterparts.
Because this process is detrimental to many prospective applications
of the QDs, the development of effective approaches for suppressing
Auger recombination has been an important goal in the QD field. One
such approach involves âsmoothingâ of the confinement
potential, which suppresses the intraband transition involved in the
dissipation of the electronâhole recombination energy. The
present study evaluates the effect of increasing âsmoothnessâ
of the confinement potential on Auger decay employing a series of
CdSe/CdS-based QDs wherein the core and the shell are separated by
an intermediate layer of a CdSe<sub><i>x</i></sub>S<sub>1â<i>x</i></sub> alloy comprised of 1â5 sublayers
with a radially tuned composition. As inferred from single-dot measurements,
use of the five-step grading scheme allows for strong suppression
of Auger decay for both biexcitons and charged excitons. Further,
due to nearly identical emissivities of neutral and charged excitons,
these QDs exhibit an interesting phenomenon of lifetime blinking for
which random fluctuations of a photoluminescence lifetime occur for
a nearly constant emission intensity
Effect of Auger Recombination on Lasing in Heterostructured Quantum Dots with Engineered Core/Shell Interfaces
Nanocrystal
quantum dots (QDs) are attractive materials for applications as laser
media because of their bright, size-tunable emission and the flexibility
afforded by colloidal synthesis. Nonradiative Auger recombination,
however, hampers optical amplification in QDs by rapidly depleting
the population of gain-active multiexciton states. In order to elucidate
the role of Auger recombination in QD lasing and isolate its influence
from other factors that might affect optical gain, we study two types
of CdSe/CdS core/shell QDs with the same core radii and the same total
sizes but different properties of the core/shell interface (âsharpâ
vs âsmoothâ). These samples exhibit distinctly different
biexciton Auger lifetimes but are otherwise virtually identical. The
suppression of Auger recombination in the sample with a smooth (alloyed)
interface results in a notable improvement in the optical gain performance
manifested in the reduction of the threshold for amplified spontaneous
emission and the ability to produce dual-color lasing involving both
the band-edge (1S) and the higher-energy (1P) electronic states. We
develop a model, which explicitly accounts for the multiexciton nature
of optical gain in QDs, and use it to analyze the competition between
stimulated emission from multiexcitons and their decay via Auger recombination.
These studies re-emphasize the importance of Auger recombination control
for the realization of real-life QD-based lasing technologies and
offer practical strategies for suppression of Auger recombination
via âinterface engineeringâ in core/shell structures
Effect of the Core/Shell Interface on Auger Recombination Evaluated by Single-Quantum-Dot Spectroscopy
Previous single-particle spectroscopic
studies of colloidal quantum
dots have indicated a significant spread in biexciton lifetimes across
an ensemble of nominally identical nanocrystals. It has been speculated
that in addition to dot-to-dot variation in physical dimensions, this
spread is contributed to by variations in the structure of the quantum
dot interface, which controls the shape of the confinement potential.
Here, we directly evaluate the effect of the composition of the coreâshell
interface on single- and multiexciton dynamics via side-by-side measurements
of individual coreâshell CdSe/CdS nanocrystals with a sharp
versus smooth (graded) interface. To realize the latter type of structures
we incorporate a CdSe<sub><i>x</i></sub>S<sub>1â<i>x</i></sub> alloy layer of controlled composition and thickness
between the CdSe core and the CdS shell. We observe that while having
essentially no effect on single-exciton decay, the interfacial alloy
layer leads to a systematic increase in biexciton lifetimes, which
correlates with the increase in the biexciton emission efficiency,
as inferred from two-photon correlation measurements. These observations
provide direct experimental evidence that in addition to the size
of the quantum dot, its interfacial properties also significantly
affect the rate of Auger recombination, which governs biexciton decay.
These findings help rationalize previous observations of a significant
heterogeneity in the biexciton lifetimes across similarly sized quantum
dots and should facilitate the development of âAuger-recombination-freeâ
colloidal nanostructures for a range of applications from lasers and
light-emitting diodes to photodetectors and solar cells
Tailored Electronic Structure and Optical Properties of Conjugated Systems through Aggregates and DipoleâDipole Interactions
A series of PPVO (<i>p</i>-phenylene vinylene oligomer) derivatives with functional groups
of varying electronegativity were synthesized via the HornerâWadsworthâEmmons
reaction. Subtle changes in the end group functionality significantly
impact the molecular electronic and optical properties of the PPVOs,
resulting in broadly tunable and efficient UV absorption and photoluminescence
spectra. Of particular interest is the NO<sub>2</sub>-substituted
PPVO which exhibits photoluminescence color ranging from the blue
to the red, thus encompassing the entire visible spectrum. Our experimental
study and electronic structure calculations suggest that the formation
of aggregates and strong dipoleâdipole soluteâsolvent
interactions are responsible for the observed strong solvatochromism.
Experimental and theoretical results for the NH<sub>2</sub>-, H-,
and NO<sub>2</sub>-substituted PPVOs suggest that the stabilization
of ground or excited state dipoles leads to the blue or red shift
of the optical spectra. The electroluminescence (EL) spectra of H-,
COOH-, and NO<sub>2</sub>-PPVO have maxima at 487, 518, and 587 nm,
respectively, in the OLED device. This trend in the EL spectra is
in excellent agreement with the end group-dependent PL spectra of
the PPVO thin-films
Single-Nanocrystal Photoluminescence Spectroscopy Studies of PlasmonâMultiexciton Interactions at Low Temperature
Using thick-shell or âgiantâ
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
Controlled Alloying of the CoreâShell Interface in CdSe/CdS Quantum Dots for Suppression of Auger Recombination
The influence of a CdSe<sub><i>x</i></sub>S<sub>1â<i>x</i></sub> interfacial alloyed layer on the photophysical properties of core/shell CdSe/CdS nanocrystal quantum dots (QDs) is investigated by comparing reference QDs with a sharp core/shell interface to alloyed structures with an intermediate CdSe<sub><i>x</i></sub>S<sub>1â<i>x</i></sub> layer at the core/shell interface. To fully realize the structural contrast, we have developed two novel synthetic approaches: a method for fast CdS-shell growth, which results in an abrupt core/shell boundary (no intentional or unintentional alloying), and a method for depositing a CdSe<sub><i>x</i></sub>S<sub>1â<i>x</i></sub> alloy layer of controlled composition onto the CdSe core prior to the growth of the CdS shell. Both types of QDs possess similar size-dependent single-exciton properties (photoluminescence energy, quantum yield, and decay lifetime). However the alloyed QDs show a significantly longer biexciton lifetime and up to a 3-fold increase in the biexciton emission efficiency compared to the reference samples. These results provide direct evidence that the structure of the QD interface has a significant effect on the rate of nonradiative Auger recombination, which dominates biexciton decay. We also observe that the energy gradient at the coreâshell interface introduced by the alloyed layer accelerates hole trapping from the shell to the core states, which results in suppression of shell emission. This comparative study offers practical guidelines for controlling multicarrier Auger recombination without a significant effect on either spectral or dynamical properties of single excitons. The proposed strategy should be applicable to QDs of a variety of compositions (including, <i>e.g</i>., infrared-emitting QDs) and can benefit numerous applications from light emitting diodes and lasers to photodetectors and photovoltaics
Thick-Shell CuInS<sub>2</sub>/ZnS Quantum Dots with Suppressed âBlinkingâ and Narrow Single-Particle Emission Line Widths
Quantum
dots (QDs) of ternary IâIIIâVI<sub>2</sub> compounds
such as CuInS<sub>2</sub> and CuInSe<sub>2</sub> have been actively
investigated as heavy-metal-free alternatives to cadmium- and lead-containing
semiconductor nanomaterials. One serious limitation of these nanostructures,
however, is a large photoluminescence (PL) line width (typically >300
meV), the origin of which is still not fully understood. It remains
even unclear whether the observed broadening results from considerable
sample heterogeneities (due, e.g., to size polydispersity) or is an
unavoidable intrinsic property of individual QDs. Here, we answer
this question by conducting single-particle measurements on a new
type of CuInS<sub>2</sub> (CIS) QDs with an especially thick ZnS shell.
These QDs show a greatly enhanced photostability compared to core-only
or thin-shell samples and, importantly, exhibit a strongly suppressed
PL blinking at the single-dot level. Spectrally resolved measurements
reveal that the single-dot, room-temperature PL line width is much
narrower (down to âŒ60 meV) than that of the ensemble samples.
To explain this distinction, we invoke a model wherein PL from CIS
QDs arises from radiative recombination of a delocalized band-edge
electron and a localized hole residing on a Cu-related defect and
also account for the effects of electronâhole Coulomb coupling.
We show that random positioning of the emitting center in the QD can
lead to more than 300 meV variation in the PL energy, which represents
at least one of the reasons for large PL broadening of the ensemble
samples. These results suggest that in addition to narrowing size
dispersion, future efforts on tightening the emission spectra of these
QDs might also attempt decreasing the âpositionalâ heterogeneity
of the emitting centers
Impact of Morphological Inhomogeneity on Excitonic States in Highly Mismatched Alloy ZnSe<sub>1â<i>X</i></sub>Te<sub><i>X</i></sub> Nanocrystals
ZnSe1âXTeX nanocrystals (NCs) are promising photon emitters
with tunable
emission across the violet to orange range and near-unity quantum
yields. However, these NCs suffer from broad emission line widths
and multiple exciton decay dynamics, which discourage their practicable
use. Here, we explore the excitonic states in ZnSe1âXTeX NCs and their photophysical
characteristics in relation to the morphological inhomogeneity of
highly mismatched alloys. Ensemble and single-dot spectroscopic analysis
of a series of ZnSe1âXTeX NC samples with varying Te ratios coupled with computational
calculations shows that, due to the distinct electronegativity between
Se and Te, nearest-neighbor Te pairs in ZnSe1âXTeX alloys create localized
hole states spectrally distributed approximately 130 meV above the
1Sh level of homogeneous ZnSe1âXTeX NCs. This forms spatially separated
excitons (delocalized electron and localized hole in trap), accounting
for both inhomogeneous and homogeneous line width broadening with
delayed recombination dynamics. Our results identify photophysical
characteristics of excitonic states in NCs made of highly mismatched
alloys and provide future research directions with potential implications
for photonic applications