16 research outputs found
Dangling Bond Defects: The Critical Roadblock to Efficient Photoconversion in Hybrid Quantum Dot Solar Cells
Inorganic–organic
hybrid materials based on silicon quantum
dots (SiQDs) have been utilized for photovoltaic applications but
suffer from rapid charge recombination and low carrier mobility. We
present an ab initio investigation of charge dynamics to pinpoint
the source of this severe problem, and our results indicate that such
devices show great promise provided that dangling bond (DB) defects
can be sufficiently removed. Without DBs, the predicted charge transfer
(CT) rate is much higher than that of photoluminescence (PL), while
the electron hopping (EH) proceeds more quickly than interfacial charge
recombination (CR). In contrast, one DB in a SiQD leads to a dramatic
enhancement, by 10 orders of magnitude, in the CR rate and a reduction
of the EH rate by 4 orders of magnitude, so that the diffusion of
carriers to electrodes becomes extremely difficult. Although other
factors, such as dot size distribution and oxidation, also play a
deleterious role in device performance, their effects are deemed much
less important than the critical role played by dangling bonds
Tunable and Energetically Robust PbS Nanoplatelets for Optoelectronic Applications
PbS
nanoplatelets (NPLs) are proposed as robust materials for novel
optoelectronic devices. Compared to quantum dot assemblies, ab initio
simulations are employed to show that such pseudo-two-dimensional
systems may provide stronger absorption and higher carrier mobility
due to the distinct wave function distributions, large electronic
couplings, and small hopping barriers. More importantly, both energetic
and spatial traps are absent in conditions far from charge balance,
indicating an extraordinary robustness against off-stoichiometry as
a result of surface homogeneity and sufficient cross-linking. Based
on our findings, we present several types of optoelectronic device
architectures spanning photovoltaics and photodetectors that could
take advantage of the superior properties found in NPLs
Toward the Ultimate Limit of Connectivity in Quantum Dots with High Mobility and Clean Gaps
Colloidal
quantum dots (CQDs) are highly versatile nanoscale optoelectronic
building blocks, but despite their materials engineering flexibility,
there is a considerable lack of fundamental understanding of their
electronic structure as they couple within thin films. By employing
a joint experimental–theoretical study, we reveal the impact
of connectivity in CQD assemblies, going beyond the single CQD picture.
High-resolution transmission electron microscopy (HR-TEM) demonstrates
connectivity motifs across different CQD sizes and length scales and
provides the necessary perspective to build robust computational models
to systematically study the achievable degree of connectivity in these
materials. We focused on state-of-the-art surface ligand treatments,
taking into account both the degree of connectivity and nanocrystal
orientation, and performed <i>ab initio</i> simulations
within the phonon-assisted hopping regime. Importantly, both the TEM
studies and our simulation results revealed morphological and electronic
defects that could dramatically reduce optoelectronic performance,
and yet would not have been captured within a single CQD model that
neglects connectivity. We calculate carrier mobility in the presence
of such defect states and conclude that the best-achievable CQD assemblies
for optoelectronics will require a modest degree of fusing <i>via</i> the {001} facet, followed by atomic ligand passivation
to generate a clean band gap and unprecedentedly high charge transport
Room Temperature Multiferroicity of Charge Transfer Crystals
Room temperature multiferroics has been a frontier research field by manipulating spin-driven ferroelectricity or charge-order-driven magnetism. Charge-transfer crystals based on electron donor and acceptor assembly, exhibiting simultaneous spin ordering, are drawing significant interests for the development of all-organic magnetoelectric multiferroics. Here, we report that a remarkable anisotropic magnetization and room temperature multiferroicity can be achieved through assembly of thiophene donor and fullerene acceptor. The crystal motif directs the dimensional and compositional control of charge-transfer networks that could switch magnetization under external stimuli, thereby opening up an attractive class of all-organic nanoferronics
Epitaxial Templating of Two-Dimensional Metal Chloride Nanocrystals on Monolayer Molybdenum Disulfide
We demonstrate the formation of ionic
metal chloride (CuCl) two-dimensional
(2D) nanocrystals epitaxially templated on the surface of monolayer
molybdenum disulfide (MoS<sub>2</sub>). These 2D CuCl nanocrystals
are single atomic planes from a nonlayered bulk CuCl structure. They
are stabilized as a 2D monolayer on the surface of the MoS<sub>2</sub> through interactions with the uniform periodic surface of the MoS<sub>2</sub>. The heterostructure 2D system is studied at the atomic level
using aberration-corrected transmission electron microscopy at 80
kV. Dynamics of discrete rotations of the CuCl nanocrystals are observed,
maintaining two types of preferential alignments to the MoS<sub>2</sub> lattice, confirming that the strong interlayer interactions drive
the stable CuCl structure. Strain maps are produced from displacement
maps and used to track real-time variations of local atomic bonding
and defect production. Density functional theory calculations interpret
the formation of two types of energetically advantageous commensurate
superlattices <i>via</i> strong chemical bonds at interfaces
and predict their corresponding electronic structures. These results
show how vertical heterostructured 2D nanoscale systems can be formed
beyond the simple assembly of preformed layered materials and provide
indications about how different 2D components and their interfacial
coupling mode could influence the overall property of the heterostructures
Freestanding Organic Charge-Transfer Conformal Electronics
Wearable
conformal electronics are essential components for next-generation
humanlike sensing devices that can accurately respond to external
stimuli in nonplanar and dynamic surfaces. However, to explore this
potential, it is indispensable to achieve the desired level of deformability
and charge-transport mobility in strain-accommodating soft semiconductors.
Here, we show pseudo-two-dimensional freestanding conjugated polymer
heterojunction nanosheets integrated into substrate-free conformal
electronics owing to their exceptional crystalline controlled charge
transport and high level of mechanical strength. These freestanding
and mechanical robust polymer nanosheets can be adapted into a variety
of artificial structured surfaces such as fibers, squares, circles,
etc., which produce large-area stretchable conformal charge-transfer
sensors for real-time static and dynamic monitoring
Atomic Structure and Dynamics of Defects in 2D MoS<sub>2</sub> Bilayers
We present a detailed atomic-level
study of defects in bilayer
MoS<sub>2</sub> using aberration-corrected transmission electron microscopy
at an 80 kV accelerating voltage. Sulfur vacancies are found in both
the top and bottom layers in 2H- and 3R-stacked MoS<sub>2</sub> bilayers.
In 3R-stacked bilayers, sulfur vacancies can migrate between layers
but more preferably reside in the (Mo–2S) column rather than
the (2S) column, indicating more complex vacancy production and migration
in the bilayer system. As the point vacancy number increases, aggregation
into larger defect structures occurs, and this impacts the interlayer
stacking. Competition between compression in one layer from the loss
of S atoms and the van der Waals interlayer force causes much less
structural deformations than those in the monolayer system. Sulfur
vacancy lines neighboring in top and bottom layers introduce less
strain compared to those staggered in the same layer. These results
show how defect structures in multilayered two-dimensional materials
differ from their monolayer form
Atomically Flat Zigzag Edges in Monolayer MoS<sub>2</sub> by Thermal Annealing
The
edges of 2D materials show novel electronic, magnetic, and
optical properties, especially when reduced to nanoribbon widths.
Therefore, methods to create atomically flat edges in 2D materials
are essential for future exploitation. Atomically flat edges in 2D
materials are found after brittle fracture or when electrically biasing,
but a simple scalable approach for creating atomically flat periodic
edges in monolayer 2D transition metal dichalcogenides has yet to
be realized. Here, we show how heating monolayer MoS<sub>2</sub> to
800 °C in vacuum produces atomically flat Mo terminated zigzag
edges in nanoribbons. We study this at the atomic level using an ultrastable
in situ heating holder in an aberration-corrected transmission electron
microscope and discriminating Mo from S at the edge, revealing unique
Mo terminations for all zigzag orientations that remain stable and
atomically flat when cooling back to room temperature. Highly faceted
MoS<sub>2</sub> nanoribbon constrictions are produced with Mo rich
edge structures that have theoretically predicted spin separated transport
channels, which are promising for spin logic applications
Molecular Assembly-Induced Charge Transfer for Programmable Functionalities
The
donor–acceptor interface within molecular charge transfer
(CT) solids plays a vital role in the hybridization of molecular orbitals
to determine their carrier transport and electronic delocalization.
In this study, we demonstrate molecular assembly-driven bilayer and
crystalline solids, consisting of electron donor dibenzotetrathiafulvalene
(DBTTF) and acceptor C<sub>60</sub>, in which interfacial engineering-induced
CT degree control is a key parameter for tuning its optical, electronic,
and magnetic performance. Compared to the DBTTF/C<sub>60</sub> bilayer
structure, the DBTTFC<sub>60</sub> cocrystalline solids show a stronger
degree of charge transfer for broad CT absorption and a large dielectric
constant. In addition, the DBTTFC<sub>60</sub> cocrystals exhibit
distinct CT arrangement-driven anisotropic electron mobility and spin
characteristics, which further enables the development of high-penetration
and high-energy γ-ray photodetectors. The results presented
in this paper provide a basis for the design and control of molecular
charge transfer solids, which facilitates the integration of such
materials into molecular electronics
Bandgap Tuning of Silicon Quantum Dots by Surface Functionalization with Conjugated Organic Groups
The
quantum confinement and enhanced optical properties of silicon quantum
dots (SiQDs) make them attractive as an inexpensive and nontoxic material
for a variety of applications such as light emitting technologies
(lighting, displays, sensors) and photovoltaics. However, experimental
demonstration of these properties and practical application into optoelectronic
devices have been limited as SiQDs are generally passivated with covalently
bound insulating alkyl chains that limit charge transport. In this
work, we show that strategically designed triphenylamine-based surface
ligands covalently bonded to the SiQD surface using conjugated vinyl
connectivity results in a 70 nm red-shifted photoluminescence relative
to their decyl-capped control counterparts. This suggests that electron
density from the SiQD is delocalized into the surface ligands to effectively
create a larger hybrid QD with possible macroscopic charge transport
properties