42 research outputs found
Photosensitivity Enhancement with TiO<sub>2</sub> in Semitransparent Light-Sensitive Skins of Nanocrystal Monolayers
We propose and demonstrate light-sensitive
nanocrystal skins that
exhibit broadband sensitivity enhancement based on electron transfer
to a thin TiO<sub>2</sub> film grown by atomic layer deposition. In
these photosensors, which operate with no external bias, photogenerated
electrons remain trapped inside the nanocrystals. These electrons
generally recombine with the photogenerated holes that accumulate
at the top interfacing contact, which leads to lower photovoltage
buildup. Because favorable conduction band offset aids in transferring
photoelectrons from CdTe nanocrystals to the TiO<sub>2</sub> layer,
which decreases the exciton recombination probability, TiO<sub>2</sub> has been utilized as the electron-accepting material in these light-sensitive
nanocrystal skins. A controlled interface thickness between the TiO<sub>2</sub> layer and the monolayer of CdTe nanocrystals enables a photovoltage
buildup enhancement in the proposed nanostructure platform. With TiO<sub>2</sub> serving as the electron acceptor, we observed broadband sensitivity
improvement across 350ā475 nm, with an approximately 22% enhancement.
Furthermore, time-resolved fluorescence measurements verified the
electron transfer from the CdTe nanocrystals to the TiO<sub>2</sub> layer in light-sensitive skins. These results could pave the way
for engineering nanocrystal-based light-sensing platforms, such as
smart transparent windows, light-sensitive walls, and large-area optical
detection systems
Exciton Dynamics in Colloidal Quantum-Dot LEDs under Active Device Operations
Colloidal quantum-dot light-emitting
diodes (QLEDs) are lucrative
options for color-pure lighting sources. To achieve high-performance
QLEDs, besides developing high-efficiency quantum dots (QDs), it is
essential to understand their device physics. However, little understanding
of the QD emission behavior in active QLEDs is one of the main factors
hindering the improvement of device efficiency. In this work, we systematically
studied the exciton dynamics of gradient composition CdSe@ZnS QDs
during electroluminescence in a working QLED. With time-resolved photoluminescence
analyses using fluorescence lifetime imaging microscopy we analyzed
a large population of QDs spatially spreading over an extended area
inside and outside the device. This allows us to reveal the statistically
significant changes in the behavior of QD emission in the device at
different levels of applied voltages and injection currents. We find
that the QD emission efficiency first drops in device fabrication
with Al electrode deposition and that the QD exciton lifetime is then
statistically reduced further under the QLEDās working conditions.
This implies the nonradiative Auger recombination process is active
in charged QDs as a result of imbalanced charge injection in a working
QLED. Our results help to understand the exciton behavior during the
operation of a QLED and demonstrate a new approach to explore the
exciton dynamics statistically with a large QD population
FoĢrster-Type Nonradiative Energy Transfer for Assemblies of Arrayed Nanostructures: Confinement Dimension vs Stacking Dimension
FoĢrster-type nonradiative
energy transfer (NRET) provides us with the ability to transfer excitation
energy between proximal nanostructures with high efficiency under
certain conditions. Nevertheless, the well-known FoĢrster theory
was developed for the case of a single donor (e.g., a molecule, a
dye) together with single acceptor. There is no complete understanding
for the cases when the donors and the acceptors are assembled in nanostructure
arrays, though there are special cases previously studied. Thus, a
comprehensive theory that models FoĢrster-type NRET for assembled
nanostructure arrays is required. Here, we report a theoretical framework
of generalized theory for the FoĢrster-type NRET with mixed
dimensionality in arrays. These include combinations of arrayed nanostructures
made of nanoparticles (NPs) and nanowires (NWs) assemblies in one-dimension
(1D), two-dimension (2D), and three-dimension (3D) completing the
framework for the transfer rates in all possible combinations of different
confinement geometries and assembly architectures, we obtain a unified
picture of NRET in assembled nanostructures arrays. We find that the
generic NRET distance dependence is modified by arraying the nanostructures.
For an acceptor NP the rate distance dependence changes from Ī³
ā <i>d</i><sup>ā6</sup> to Ī³ ā <i>d</i><sup>ā5</sup> when they are arranged in a 1D stack,
and to Ī³ ā <i>d</i><sup>ā4</sup> when
in a 2D array, and to Ī³ ā <i>d</i><sup>ā3</sup> when in a 3D array. Likewise, an acceptor NW changes its distance
dependence from Ī³ ā <i>d</i><sup>ā5</sup> to Ī³ ā <i>d</i><sup>ā4</sup> when
they are arranged in a 1D array and to Ī³ ā <i>d</i><sup>ā3</sup> when in a 2D array. These finding shows that
the numbers of dimensions across which nanostructures are stacked
is equally critical as the confinement dimension of the nanostructure
in determining the NRET kinetics
Amplified Spontaneous Emission and Lasing in Colloidal Nanoplatelets
Colloidal nanoplatelets (NPLs) have recently emerged as favorable light-emitting materials, which also show great potential as optical gain media due to their remarkable optical properties. In this work, we systematically investigate the optical gain performance of CdSe core and CdSe/CdS core/crown NPLs having different CdS crown size with one- and two-photon absorption pumping. The core/crown NPLs exhibit enhanced gain performance as compared to the core-only NPLs due to increased absorption cross section and the efficient interexciton funneling, which is from the CdS crown to the CdSe core. One- and two-photon absorption pumped amplified spontaneous emission thresholds are found as low as 41 Ī¼J/cm<sup>2</sup> and 4.48 mJ/cm<sup>2</sup>, respectively. These thresholds surpass the best reported optical gain performance of the state-of-the-art colloidal nanocrystals (<i>i.e.</i>, quantum dots, nanorods, <i>etc.</i>) emitting in the same spectral range as the NPLs. Moreover, gain coefficient of the NPLs is measured as high as 650 cm<sup>ā1</sup>, which is 4-fold larger than the best reported gain coefficient of the colloidal quantum dots. Finally, we demonstrate a two-photon absorption pumped vertical cavity surface emitting laser of the NPLs with a lasing threshold as low as 2.49 mJ/cm<sup>2</sup>. These excellent results are attributed to the superior properties of the NPLs as optical gain media
Temperature-Dependent Emission Kinetics of Colloidal Semiconductor Nanoplatelets Strongly Modified by Stacking
We
systematically studied temperature-dependent emission kinetics
in solid films of solution-processed CdSe nanoplatelets (NPLs) that
are either intentionally stacked or nonstacked. We observed that the
steady-state photoluminescence (PL) intensity of nonstacked NPLs considerably
increases with decreasing temperature, whereas there is only a slight
increase in stacked NPLs. Furthermore, PL decay time of the stacked
NPL ensemble is comparatively much shorter than that of the nonstacked
NPLs, and this result is consistent at all temperatures. To account
for these observations, we developed a probabilistic model that describes
excitonic processes in a stack using Markov chains, and we found excellent
agreement between the model and experimental results. These findings
develop the insight that the competition between the radiative channels
and energy transfer-assisted hole trapping leads to weakly temperature-dependent
PL intensity in the case of the stacked NPL ensembles as compared
to the nonstacked NPLs lacking strong energy transfer. This study
shows that it is essential to account for the effect of NPL stacking
to understand their resulting PL emission properties
Stacking in Colloidal Nanoplatelets: Tuning Excitonic Properties
Colloidal semiconductor quantum wells, also commonly known as nanoplatelets (NPLs), have arisen among the most promising materials for light generation and harvesting applications. Recently, NPLs have been found to assemble in stacks. However, their emerging characteristics essential to these applications have not been previously controlled or understood. In this report, we systematically investigate and present excitonic properties of controlled column-like NPL assemblies. Here, by a controlled gradual process, we show that stacking in colloidal quantum wells substantially increases exciton transfer and trapping. As NPLs form into stacks, surprisingly we find an order of magnitude decrease in their photoluminescence quantum yield, while the transient fluorescence decay is considerably accelerated. These observations are corroborated by ultraefficient FoĢrster resonance energy transfer (FRET) in the stacked NPLs, in which exciton migration is estimated to be in the ultralong range (>100 nm). Homo-FRET (<i>i</i>.<i>e</i>., FRET among the same emitters) is found to be ultraefficient, reaching levels as high as 99.9% at room temperature owing to the close-packed collinear orientation of the NPLs along with their large extinction coefficient and small Stokes shift, resulting in a large FoĢrster radius of ā¼13.5 nm. Consequently, the strong and long-range homo-FRET boosts exciton trapping in nonemissive NPLs, acting as exciton sink centers, quenching photoluminescence from the stacked NPLs due to rapid nonradiative recombination of the trapped excitons. The rate-equation-based model, which considers the exciton transfer and the radiative and nonradiative recombination within the stacks, shows an excellent match with the experimental data. These results show the critical significance of stacking control in NPL solids, which exhibit completely different signatures of homo-FRET as compared to that in colloidal nanocrystals due to the absence of inhomogeneous broadening
Plasmon-Enhanced Energy Transfer in Photosensitive Nanocrystal Device
FoĢrster
resonance energy transfer (FRET) interacted with
localized surface plasmon (LSP) gives us the ability to overcome inadequate
transfer of energy between donor and acceptor nanocrystals (NCs).
In this paper, we show LSP-enhanced FRET in colloidal photosensors
of NCs in operation, resulting in substantially enhanced photosensitivity.
The proposed photosensitive device is a layered self-assembled colloidal
platform consisting of separated monolayers of the donor and the acceptor
colloidal NCs with an intermediate metal nanoparticle (MNP) layer
made of gold interspaced by polyelectrolyte layers. Using LBL assembly,
we fabricated and comparatively studied seven types of such NC-monolayer
devices (containing only donor, only acceptor, Au MNPādonor,
Au MNPāacceptor, donorāacceptor bilayer, donorāAu
MNPāacceptor trilayer, and acceptorāAu MNPādonor
reverse trilayer). In these structures, we revealed the effect of
LSP-enhanced FRET and exciton interactions from the donor NCs layer
to the acceptor NCs layer. Compared to a single acceptor NC device,
we observed a significant extension in operating wavelength range
and a substantial photosensitivity enhancement (2.91-fold) around
the LSP resonance peak of Au MNPs in the LSP-enhanced FRET trilayer
structure. Moreover, we present a theoretical model for the intercoupled
donorāAu MNPāacceptor structure subject to the plasmon-mediated
nonradiative energy transfer. The obtained numerical results are in
excellent agreement with the systematic experimental studies done
in our work. The potential to modify the energy transfer through mastering
the excitonāplasmon interactions and its implication in devices
make them attractive for applications in nanophotonic devices and
sensors
Tunable White-Light-Emitting Mn-Doped ZnSe Nanocrystals
We report white-light-emitting Mn-doped
ZnSe nanocrystals (NCs) that are synthesized using modified nucleation
doping strategy. Tailoring three distinct emission mechanisms in these
NCs, which are MnSe-related blue emission (410 and 435 nm), Zn-related
defect state green emission (520 nm), and Mn-dopant related orange
emission (580 nm), allowed us to achieve excitation wavelength tailorable
white-light generation as studied by steady state and time-resolved
fluorescence spectroscopy. These NCs will be promising as single component
white-light engines for solid-state lighting
Solvent-Assisted Surface Engineering for High-Performance All-Inorganic Perovskite Nanocrystal Light-Emitting Diodes
All-inorganic
cesium halide perovskite nanocrystals have attracted much interest
in optoelectronic applications for the sake of the readily adjustable
band gaps, high photoluminescence quantum yield, pure color emission,
and affordable cost. However, because of the ineluctable utilization
of organic surfactants during the synthesis, the structural and optical
properties of CsPbBr<sub>3</sub> nanocrystals degrade upon transforming
from colloidal solutions to solid thin films, which plagues the device
operation. Here, we develop a novel solvent-assisted surface engineering
strategy, producing high-quality CsPbBr<sub>3</sub> thin films for
device applications. A good solvent is first introduced as an assembly
trigger to conduct assembly in a one-dimensional direction, which
is then interrupted by adding a nonsolvent. The nonsolvent drives
the adjacent nanoparticles connecting in a two-dimensional direction.
Assembled CsPbBr<sub>3</sub> nanocrystal thin films are densely packed
and very smooth with a surface roughness of ā¼4.8 nm, which
is highly desirable for carrier transport in a light-emitting diode
(LED) device. Meanwhile, the film stability is apparently improved.
Benefiting from this facile and reliable strategy, we have achieved
remarkably improved performance of CsPbBr<sub>3</sub> nanocrystal-based
LEDs. Our results not only enrich the methods of nanocrystal surface
engineering but also shed light on developing high-performance LEDs
Phonon-Assisted Exciton Transfer into Silicon Using Nanoemitters: The Role of Phonons and Temperature Effects in FoĢrster Resonance Energy Transfer
We study phonon-assisted FoĢrster resonance energy transfer (FRET) into an indirect band-gap semiconductor using nanoemitters. The unusual temperature dependence of this energy transfer, which is measured using the donor nanoemitters of quantum dot (QD) layers integrated on the acceptor monocrystalline bulk silicon as a model system, is predicted by a phonon-assisted exciton transfer model proposed here. The model includes the phonon-mediated optical properties of silicon, while considering the contribution from the multimonolayer-equivalent QD film to the nonradiative energy transfer, which is derived with a <i>d</i><sup>ā3</sup> distance dependence. The FRET efficiencies are experimentally observed to decrease at cryogenic temperatures, which are well explained by the model considering the phonon depopulation in the indirect band-gap acceptor together with the changes in the quantum yield of the donor. These understandings will be crucial for designing FRET-enabled sensitization of silicon based high-efficiency excitonic systems using nanoemitters