9 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 ā¼6% on average. We analyze limitations of these quantum dots associated with irreversible photodegradation and fluctuations (āblinkingā) of the photoluminescence intensity. On the basis of emission intensity-lifetime correlations, we assign the āblinkingā 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
Effect of Interfacial Alloying versus āVolume Scalingā on Auger Recombination in Compositionally Graded Semiconductor Quantum Dots
Auger recombination
is a nonradiative three-particle process wherein
the electronāhole 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 āsmoothingā of the confinement
potential, which suppresses the intraband transition involved in the
dissipation of the electronāhole recombination energy. The
present study evaluates the effect of increasing āsmoothnessā
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>1ā<i>x</i></sub> alloy comprised of 1ā5 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
Light Emission Mechanisms in CuInS<sub>2</sub> Quantum Dots Evaluated by Spectral Electrochemistry
Luminescent
CuInS<sub>2</sub> (CIS) quantum dots (QDs) exhibit
highly efficient intragap emission and long, hundreds-of-nanoseconds
radiative lifetimes. These spectral properties, distinct from structurally
similar IIāVI QDs, can be explained by the involvement of intragap
defect states containing a localized hole capable of coupling with
a conduction band electron for a radiative transition. However, the
absolute energies of the intragap and band-edge states, the structure
of the emissive defect(s), and the role and origin of nonemissive
decay channels still remain poorly understood. Here, we address these
questions by applying methods of spectral electrochemistry. Cyclic
voltammetry measurements reveal a well-defined intragap state whose
redox potential is close to that of the Cu<sup><i>x</i></sup> defect state (where <i>x</i> = 1+ or 2+). The energy offset
of this state from the valence band accounts well for the apparent
photoluminescence Stokes shift observed in optical spectra. These
results provide direct evidence that Cu-related defects serve as emission
centers responsible for strong intragap emission from CIS QDs. We
then use <i>in situ</i> spectroelectrochemistry to reveal
two distinct emission pathways based on the differing oxidation states
of Cu defects, which can be controlled by altering QD stoichiometry
(1+ for stoichiometric QDs and 2+ for Cu-deficient QDs)
High-Performance CuInS<sub>2</sub> Quantum Dot Laminated Glass Luminescent Solar Concentrators for Windows
Building-integrated
sunlight harvesting utilizing laminated glass
luminescent solar concentrators (LSCs) is proposed. By incorporating
high quantum yield (>90%), NIR-emitting CuInS<sub>2</sub>/ZnS quantum
dots into the polymer interlayer between two sheets of low-iron float
glass, a record optical efficiency of 8.1% is demonstrated for a 10
cm Ć 10 cm device that transmits ā¼44% visible light. After
completing prototypes by attaching silicon solar cells along the perimeter
of the device, the electrical power conversion efficiency was certified
at 2.2% with a black background and at 2.9% using a reflective substrate.
This ādrop-inā LSC solution is particularly attractive
because it fits within the existing glazing industry value chain with
only modest changes to typical glazing products. Performance modeling
predicts >1 GWh annual electricity production for a typical urban
skyscraper in most major U.S. cities, enabling significant energy
cost savings and potentially ānet-zeroā buildings
Spectral and Dynamical Properties of Single Excitons, Biexcitons, and Trions in CesiumāLead-Halide Perovskite Quantum Dots
Organicāinorganic lead-halide
perovskites have been the subject of recent intense interest due to
their unusually strong photovoltaic performance. A new addition to
the perovskite family is all-inorganic CsāPb-halide perovskite
nanocrystals, or quantum dots, fabricated via a moderate-temperature
colloidal synthesis. While being only recently introduced to the research
community, these nanomaterials have already shown promise for a range
of applications from color-converting phosphors and light-emitting
diodes to lasers, and even room-temperature single-photon sources.
Knowledge of the optical properties of perovskite quantum dots still
remains vastly incomplete. Here we apply various time-resolved spectroscopic
techniques to conduct a comprehensive study of spectral and dynamical
characteristics of single- and multiexciton states in CsPbX<sub>3</sub> nanocrystals with X being either Br, I, or their mixture. Specifically,
we measure exciton radiative lifetimes, absorption cross-sections,
and derive the degeneracies of the band-edge electron and hole states.
We also characterize the rates of intraband cooling and nonradiative
Auger recombination and evaluate the strength of excitonāexciton
coupling. The overall conclusion of this work is that spectroscopic
properties of CsāPb-halide quantum dots are largely similar
to those of quantum dots of more traditional semiconductors such as
CdSe and PbSe. At the same time, we observe some distinctions including,
for example, an appreciable effect of the halide identity on radiative
lifetimes, considerably shorter biexciton Auger lifetimes, and apparent
deviation of their size dependence from the āuniversal volume
scalingā previously observed for many traditional nanocrystal
systems. The high efficiency of Auger decay in perovskite quantum
dots is detrimental to their prospective applications in light-emitting
devices and lasers. This points toward the need for the development
of approaches for effective suppression of Auger recombination in
these nanomaterials, using perhaps insights gained from previous studies
of IIāVI nanocrystals
Thick-Shell CuInS<sub>2</sub>/ZnS Quantum Dots with Suppressed āBlinkingā and Narrow Single-Particle Emission Line Widths
Quantum
dots (QDs) of ternary IāIIIāVI<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 ā¼60 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 electronāhole 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 āpositionalā heterogeneity
of the emitting centers
Design and Synthesis of Heterostructured Quantum Dots with Dual Emission in the Visible and Infrared
The unique optical properties exhibited by visible emitting core/shell quantum dots with especially thick shells are the focus of widespread study, but have yet to be realized in infrared (IR)-active nanostructures. We apply an effective-mass model to identify PbSe/CdSe core/shell quantum dots as a promising system for achieving this goal. We then synthesize colloidal PbSe/CdSe quantum dots with shell thicknesses of up to 4 nm that exhibit unusually slow hole intraband relaxation from shell to core states, as evidenced by the emergence of dual emission, <i>i</i>.<i>e</i>., IR photoluminescence from the PbSe core observed simultaneously with visible emission from the CdSe shell. In addition to the large shell thickness, the development of slowed intraband relaxation is facilitated by the existence of a sharp coreāshell interface without discernible alloying. Growth of thick shells without interfacial alloying or incidental formation of homogeneous CdSe nanocrystals was accomplished using insights attained <i>via</i> a systematic study of the dynamics of the cation-exchange synthesis of both PbSe/CdSe and the related system PbS/CdS. Finally, we show that the efficiency of the visible photoluminescence can be greatly enhanced by inorganic passivation
Two-Photon Absorption in CdSe Colloidal Quantum Dots Compared to Organic Molecules
We discuss fundamental differences in electronic structure as reflected in one- and two-photon absorption spectra of semiconductor quantum dots and organic molecules by performing systematic experimental and theoretical studies of the size-dependent spectra of colloidal quantum dots. Quantum-chemical and effective-mass calculations are used to model the one- and two-photon absorption spectra and compare them with the experimental results. Currently, quantum-chemical calculations are limited to only small-sized quantum dots (nanoclusters) but allow one to study various environmental effects on the optical spectra such as solvation and various surface functionalizations. The effective-mass calculations, on the other hand, are applicable to the larger-sized quantum dots and can, in general, explain the observed trends but are insensitive to solvent and ligand effects. Careful comparison of the experimental and theoretical results allows for quantifying the range of applicability of theoretical methods used in this work. Our study shows that the small clusters can be in principle described in a manner similar to that used for organic molecules. In addition, there are several important factors (quality of passivation, nature of the ligands, and intraband/interband transitions) affecting optical properties of the nanoclusters. The larger-size quantum dots, on the other hand, behave similarly to bulk semiconductors, and can be well described in terms of the effective-mass models
Phase-Transfer Ligand Exchange of Lead Chalcogenide Quantum Dots for Direct Deposition of Thick, Highly Conductive Films
The
use of semiconductor nanocrystal quantum dots (QDs) in optoelectronic
devices typically requires postsynthetic chemical surface treatments
to enhance electronic coupling between QDs and allow for efficient
charge transport in QD films. Despite their importance in solar cells
and infrared (IR) light-emitting diodes and photodetectors, advances
in these chemical treatments for lead chalcogenide (PbE; E = S, Se,
Te) QDs have lagged behind those of, for instance, IIāVI semiconductor
QDs. Here, we introduce a method for fast and effective ligand exchange
for PbE QDs in solution, resulting in QDs completely passivated by
a wide range of small anionic ligands. Due to electrostatic stabilization,
these QDs are readily dispersible in polar solvents, in which they
form highly concentrated solutions that remain stable for months.
QDs of all three Pb chalcogenides retain their photoluminescence,
allowing for a detailed study of the effect of the surface ionic double
layer on electronic passivation of QD surfaces, which we find can
be explained using the hard/soft acidābase theory. Importantly,
we prepare highly conductive films of PbS, PbSe, and PbTe QDs by directly
casting from solution without further chemical treatment, as determined
by field-effect transistor measurements. This method allows for precise
control over the surface chemistry, and therefore the transport properties
of deposited films. It also permits single-step deposition of films
of unprecedented thickness via continuous processing techniques, as
we demonstrate by preparing a dense, smooth, 5.3-Ī¼m-thick PbSe
QD film via doctor-blading. As such, it offers important advantages
over laborious layer-by-layer methods for solar cells and photodetectors,
while opening the door to new possibilities in ionizing-radiation
detectors