10 research outputs found
Quantum Dot/Light-Emitting Electrochemical Cell Hybrid Device and Mechanism of Its Operation
A new type of light-emitting
hybrid device based on colloidal quantum dots (QDs) and an ionic transition
metal complex (iTMC) light-emitting electrochemical cell (LEC) is
introduced. The developed hybrid devices show light emission from
both active layers, which are combined in a stacked geometry. Time-resolved
photoluminescence experiments indicate that the emission is controlled
by direct charge injection into both the iTMC and the QD layer. The
turn-on time (time to reach 1 cd/m<sup>2</sup>) at constant voltage
operation is significantly reduced from 8 min in the case of the reference
LEC down to subsecond in the case of the hybrid device. Furthermore,
luminance and efficiency of the hybrid device are enhanced compared
to reference LEC directly after device turn-on by a factor of 400
and 650, respectively. We attribute these improvements to an increased
electron injection efficiency into the iTMC directly after device
turn-on
Solution-Processed CuInS<sub>2</sub>âBased White QD-LEDs with Mixed Active Layer Architecture
Colloidal
quantum dots (QDs) are attractive candidates for future lighting technology.
However, in contrast to display applications, the realization of balanced
white lighting devices remains conceptually challenging. Here, we
demonstrate two-component white light-emitting QD-LEDs with high color
rendering indices (CRI) up to 78. The implementation of orange CuInS<sub>2</sub>/ZnS (CIS/ZnS) QDs with a broad emission and high quantum
yield together with blue ZnCdSe/ZnS QDs in a mixed approach allowed
white light emission with low blue QD content. The devices reveal
only a small color drift in a wide operation voltage range. The correlated
color temperature (CCT) could be adjusted between 2200 and 7200 K
(from warm white to cold white) by changing the volume ratio between
orange and blue QDs (1:0.5 and 1:2)
Chemical Synthesis, Doping, and Transformation of Magic-Sized Semiconductor Alloy Nanoclusters
Nanoclusters are
important prenucleation intermediates for colloidal
nanocrystal synthesis. In addition, they exhibit many intriguing properties
originating from their extremely small size lying between molecules
and typical nanocrystals. However, synthetic control of multicomponent
semiconductor nanoclusters remains a daunting goal. Here, we report
on the synthesis, doping, and transformation of multielement magic-sized
clusters, generating the smallest semiconductor alloys. We use Lewis
acidâbase reactions at room temperature to synthesize alloy
clusters containing three or four types of atoms. Mass spectrometry
reveals that the alloy clusters exhibit âmagic-sizeâ
characteristics with chemical formula of Zn<sub><i>x</i></sub>Cd<sub>13â<i>x</i></sub>Se<sub>13</sub> (<i>x</i> = 0â13) whose compositions are tunable between
CdSe and ZnSe. Successful doping of these clusters creates a new class
of diluted magnetic semiconductors in the extreme quantum confinement
regime. Furthermore, the important role of these alloy clusters as
prenucleation intermediates is demonstrated by low temperature transformation
into quantum alloy nanoribbons and nanorods. Our study will facilitate
the understanding of these novel diluted magnetic semiconductor nanoclusters,
and offer new possibilities for the controlled synthesis of nanomaterials
at the prenucleation stage, consequently producing novel multicomponent
nanomaterials that are difficult to synthesize
Valence-Band Mixing Effects in the Upper-Excited-State Magneto-Optical Responses of Colloidal Mn<sup>2+</sup>-Doped CdSe Quantum Dots
We present an experimental study of the magneto-optical activity of multiple excited excitonic states of manganese-doped CdSe quantum dots chemically prepared by the diffusion doping method. Giant excitonic Zeeman splittings of each of these excited states can be extracted for a series of quantum dot sizes and are found to depend on the radial quantum number of the hole envelope function involved in each transition. As seven out of eight transitions involve the same electron energy state, 1S<sub>e</sub>, the dominant hole character of each excitonic transition can be identified, making use of the fact that the <i>g</i>-factor of the pure heavy-hole component has a different sign compared to pure light hole or split-off components. Because the magnetic exchange interactions are sensitive to hole state mixing, the giant Zeeman splittings reported here provide clear experimental evidence of quantum-size-induced mixing among valence-band states in nanocrystals
Digital Doping in Magic-Sized CdSe Clusters
Magic-sized
semiconductor clusters represent an exciting class
of materials located at the boundary between quantum dots and molecules.
It is expected that replacing single atoms of the host crystal with
individual dopants in a one-by-one fashion can lead to unique modifications
of the material properties. Here, we demonstrate the dependence of
the magneto-optical response of (CdSe)<sub>13</sub> clusters on the
discrete number of Mn<sup>2+</sup> ion dopants. Using time-of-flight
mass spectrometry, we are able to distinguish undoped, monodoped,
and bidoped cluster species, allowing for an extraction of the relative
amount of each species for a specific average doping concentration.
A giant magneto-optical response is observed up to room temperature
with clear evidence that exclusively monodoped clusters are magneto-optically
active, whereas the Mn<sup>2+</sup> ions in bidoped clusters couple
antiferromagnetically and are magneto-optically passive. Mn<sup>2+</sup>-doped clusters therefore represent a system where magneto-optical
functionality is caused by solitary dopants, which might be beneficial
for future solotronic applications
Current-Induced Magnetic Polarons in a Colloidal Quantum-Dot Device
Electrical
spin manipulation remains a central challenge for the realization
of diverse spin-based information processing technologies. Motivated
by the demonstration of confinement-enhanced spâd exchange
interactions in colloidal diluted magnetic semiconductor (DMS) quantum
dots (QDs), such materials are considered promising candidates for
future spintronic or spin-photonic applications. Despite intense research
into DMS QDs, electrical control of their magnetic and magneto-optical
properties remains a daunting goal. Here, we report the first demonstration
of electrically induced magnetic polaron formation in any DMS, achieved
by embedding Mn<sup>2+</sup>-doped CdSe/CdS core/shell QDs as the
active layer in an electrical light-emitting device. Tracing the electroluminescence
from cryogenic to room temperatures reveals an anomalous energy shift
that reflects current-induced magnetization of the Mn<sup>2+</sup> spin sublattice, that is, excitonic magnetic polaron formation.
These electrically induced magnetic polarons exhibit an energy gain
comparable to their optically excited counterparts, demonstrating
that magnetic polaron formation is achievable by current injection
in a solid-state device
spâd Exchange Interactions in Wave Function Engineered Colloidal CdSe/Mn:CdS Hetero-Nanoplatelets
In
two-dimensional (2D) colloidal semiconductor nanoplatelets,
which are atomically flat nanocrystals, the precise control of thickness
and composition on the atomic scale allows for the synthesis of heterostructures
with well-defined electron and hole wave function distributions. Introducing
transition metal dopants with a monolayer precision enables tailored
magnetic exchange interactions between dopants and band states. Here,
we use the absorption based technique of magnetic circular dichroism
(MCD) to directly prove the exchange coupling of magnetic dopants
with the band charge carriers in hetero-nanoplatelets with CdSe core
and manganese-doped CdS shell (CdSe/Mn:CdS). We show that the strength
of both the electron as well as the hole exchange interactions with
the dopants can be tuned by varying the nanoplatelets architecture
with monolayer accuracy. As MCD is highly sensitive for excitonic
resonances, excited level spectroscopy allows us to resolve and identify,
in combination with wave function calculations, several excited state
transitions including spinâorbit split-off excitonic contributions.
Thus, our study not only demonstrates the possibility to expand the
extraordinary physical properties of colloidal nanoplatelets toward
magneto-optical functionality by transition metal doping but also
provides an insight into the excited state electronic structure in
this novel two-dimensional material
Photogating through Unidirectional Charge Carrier Funneling in Two-Dimensional Transition Metal Dichalcogenide/Perovskite Heterostructure Photodetectors
Two-dimensional (2D) van der Waals (vdW) semiconductors
such as
transition metal dichalcogenides (TMDCs) or 2D halide perovskites
receive increasing attention as active materials in photosensing applications
due to their high oscillator strength, large electronic mobility,
and mechanical flexibility. For triggering an efficient separation
of optically generated charge carriers and hence improving the photodetectivity,
different vdW semiconductors are combined into functional heterostructures,
i.e., TMDCs and 2D RuddlesdenâPopper perovskite. However, despite
their increasing usage in devices, energy and charge carrier transfer
between TMDCs and 2D RuddlesdenâPopper materials is still controversially
discussed, and the underlying mechanisms of device operation are ambiguous.
Here, in molybdenum disulfide/butylammonium lead iodide (MoS2/BA2PbI4) heterostructures, we demonstrate
a unidirectional hole transfer from MoS2 to BA2PbI4 and an electron blocking through the butylammonium
ions. MoS2/BA2PbI4 photodetectors
show drastically improved responsivities, reduced dark current, and
an increased detectivity compared to MoS2- and BA2PbI4-only devices. We provide evidence that this improvement
is related to a gain mechanism due to photogating in the MoS2 channel caused by the unidirectional hole transfer between MoS2 and the BA2PbI4
High-Speed GaN/GaInN Nanowire Array Light-Emitting Diode on Silicon(111)
The high speed onâoff performance of GaN-based
light-emitting
diodes (LEDs) grown in c-plane direction is limited by long carrier
lifetimes caused by spontaneous and piezoelectric polarization. This
work demonstrates that this limitation can be overcome by m-planar
coreâshell InGaN/GaN nanowire LEDs grown on Si(111). Time-resolved
electroluminescence studies exhibit 90â10% rise- and fall-times
of about 220 ps under GHz electrical excitation. The data underline
the potential of these devices for optical data communication in polymer
fibers and free space
The Role of Excitation Energy in Photobrightening and Photodegradation of Halide Perovskite Thin Films
We
study the impact of excitation energy on the photostability
of methylammonium lead triiodide (CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> or MAPI) perovskite thin films. Light soaking leads to a
transient increase of the photoluminescence efficiency at excitation
wavelengths longer than 520 nm, whereas light-induced degradation
occurs when exciting the films with wavelengths shorter than 520 nm.
X-ray diffraction and extinction measurements reveal the light-induced
decomposition of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> to lead
iodide (PbI<sub>2</sub>) for the high-energy excitation regime. We
propose a model explaining the energy dependence of the photostability
that involves the photoexcitation of residual PbI<sub>2</sub> species
in the perovskite triggering the decomposition of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>