20 research outputs found
Tuning Size and Size Distribution of Colloidal InAs Nanocrystals via Continuous Supply of Prenucleation Clusters on Nanocrystal Seeds
Tuning Size and Size Distribution of Colloidal InAs
Nanocrystals via Continuous Supply of Prenucleation Clusters on Nanocrystal
Seed
Steric-Hindrance-Driven Shape Transition in PbS Quantum Dots: Understanding Size-Dependent Stability
Ambient stability of colloidal nanocrystal
quantum dots (QDs) is
imperative for low-cost, high-efficiency QD photovoltaics. We synthesized
air-stable, ultrasmall PbS QDs with diameter (<i>D</i>)
down to 1.5 nm, and found an abrupt transition at <i>D</i> ā 4 nm in the air stability as the QD size was varied from
1.5 to 7.5 nm. X-ray photoemission spectroscopy measurements and density
functional theory calculations reveal that the stability transition
is closely associated with the shape transition of oleate-capped QDs
from octahedron to cuboctahedron, driven by steric hindrance and thus
size-dependent surface energy of oleate-passivated Pb-rich QD facets.
This microscopic understanding of the surface chemistry on ultrasmall
QDs, up to a few nanometers, should be very useful for precisely and
accurately controlling physicochemical properties of colloidal QDs
such as doping polarity, carrier mobility, air stability, and hot-carrier
dynamics for solar cell applications
Steric-Hindrance-Driven Shape Transition in PbS Quantum Dots: Understanding Size-Dependent Stability
Ambient stability of colloidal nanocrystal
quantum dots (QDs) is
imperative for low-cost, high-efficiency QD photovoltaics. We synthesized
air-stable, ultrasmall PbS QDs with diameter (<i>D</i>)
down to 1.5 nm, and found an abrupt transition at <i>D</i> ā 4 nm in the air stability as the QD size was varied from
1.5 to 7.5 nm. X-ray photoemission spectroscopy measurements and density
functional theory calculations reveal that the stability transition
is closely associated with the shape transition of oleate-capped QDs
from octahedron to cuboctahedron, driven by steric hindrance and thus
size-dependent surface energy of oleate-passivated Pb-rich QD facets.
This microscopic understanding of the surface chemistry on ultrasmall
QDs, up to a few nanometers, should be very useful for precisely and
accurately controlling physicochemical properties of colloidal QDs
such as doping polarity, carrier mobility, air stability, and hot-carrier
dynamics for solar cell applications
Hysteresis and Photoinstability Caused by Mobile Ions in Colloidal Quantum Dot Photovoltaics
Organicāinorganic
hybrid photovoltaics (PVs) have recently
attracted considerable attention as their PV performance has rapidly
improved. Abnormal currentāvoltage (<i>I</i>ā<i>V</i>) characteristics or <i>I</i>ā<i>V</i> hysteresis, however, were occasionally observed in such
systems that hampered the development of the PV technology. Here we
study the hysteresis of organicāinorganic hybrid colloidal
quantum dot (CQD) PVs by analyzing <i>I</i>ā<i>V</i> characteristics upon systematic modulation of organic
components of CQDs. We demonstrate that an external bias stress to
CQD films transiently prompts redistribution of mobile ions, particularly
protons of surface ligands, thus leading to the formation of a transient
space-charge region in the CQD films. The variable space-charge region
causes <i>I</i>ā<i>V</i> hysteresis and
photoinstability of CQD PVs, which is closely correlated with the
transient behavior of mobile ions. Our findings here could provide
significant implications to the understanding of the influence of
mobile ions on <i>I</i>ā<i>V</i> hysteresis
in other organicāinorganic hybrid PVs such as perovskites
Steric-Hindrance-Driven Shape Transition in PbS Quantum Dots: Understanding Size-Dependent Stability
Ambient stability of colloidal nanocrystal
quantum dots (QDs) is
imperative for low-cost, high-efficiency QD photovoltaics. We synthesized
air-stable, ultrasmall PbS QDs with diameter (<i>D</i>)
down to 1.5 nm, and found an abrupt transition at <i>D</i> ā 4 nm in the air stability as the QD size was varied from
1.5 to 7.5 nm. X-ray photoemission spectroscopy measurements and density
functional theory calculations reveal that the stability transition
is closely associated with the shape transition of oleate-capped QDs
from octahedron to cuboctahedron, driven by steric hindrance and thus
size-dependent surface energy of oleate-passivated Pb-rich QD facets.
This microscopic understanding of the surface chemistry on ultrasmall
QDs, up to a few nanometers, should be very useful for precisely and
accurately controlling physicochemical properties of colloidal QDs
such as doping polarity, carrier mobility, air stability, and hot-carrier
dynamics for solar cell applications
Chemical Synthetic Strategy for Single-Layer Transition-Metal Chalcogenides
A solution-phase synthetic protocol
to form two-dimensional (2D)
single-layer transition-metal chalcogenides (TMCs) has long been sought;
however, such efforts have been plagued with the spontaneous formation
of multilayer sheets. In this study, we discovered a solution-phase
synthetic protocol, called ādiluted chalcogen continuous influx
(DCCI)ā, where controlling the chalcoĀgen source influx
(e.g., H<sub>2</sub>S) during its reaction with the transition-metal
halide precursor is the critical parameter for the formation of single-layer
sheets as examined for the cases of group IV TMCs. The continuous
influx of dilute H<sub>2</sub>S throughout the entire growth period
is necessary for large sheet formation through the exclusive <i>a-</i> and <i>b-</i>axial growth processes. By contrast,
the burst influx of highly concentrated H<sub>2</sub>S in the early
stages of the growth process forms multilayer TMC nanodiscs. Our DCCI
protocol is a new synthetic concept for single-layer TMCs and, in
principle, can be operative for wide range of TMC nanosheets
Size Dependence of Excitation-Energy-Related Surface Trapping Dynamics in PbS Quantum Dots
Using ultrafast transient absorption
spectroscopy, we investigated
the surface carrier trapping dynamics in various sized PbS quantum
dots (QDs) when either a hot or cold exciton is photogenerated by
different pump-energy. We observed that hot carriers exhibit distinctly
different surface trapping dynamics from the cold exciton, in which
their corresponding transient absorption (TA) spectral evolutions
show clear differences in the long wavelength region (less than a
band gap energy, <i>E</i><sub>g</sub>). We observed a rapid
growth in the degree of surface trapping with an increase in the pump-energy.
On the other hand, the degree of surface trapping in terms of the
number of created excitons (āØ<i>N</i><sub><i>x</i></sub>ā©) shows negligible variation upon photoexcitation
at any given wavelength. The photoinduced electronāhole separation
followed by carrier trapping was characterized by ultrafast trapping
rate. The surface trapping rate was solely dependent on the PbS QD
size; the surface trapping rate becomes faster as the QD size increases.
Furthermore, we explain the dependence of QD size on the surface trapping
rate in terms of the size-dependent exciton binding energy (<i>E</i><sub>eb</sub>)
Well-Defined Colloidal 2āD Layered Transition-Metal Chalcogenide Nanocrystals via Generalized Synthetic Protocols
While interesting and unprecedented material characteristics
of
two dimensionality (2-D) layered nanomaterials are emerging, their
reliable synthetic methodologies are not well developed. In this study
we demonstrate general applicability of synthetic protocols to a wide
range of colloidal 2-D layered transition-metal chalcogenide (TMC)
nanocrystals. As distinctly different from other nanocrystals, we
discovered that 2-D layered TMC nanocrystals are unstable in the presence
of reactive radicals from elemental chalcogen during the crystal formation.
We first introduce the synthesis of titanium sulfide and selenide
where well-defined single crystallinity and lateral size controllability
are verified, and then such synthetic protocols are extended to all
of group IV and V transition-metal sulfide (TiS<sub>2</sub>, ZrS<sub>2</sub>, HfS<sub>2</sub>, VS<sub>2</sub>, NbS<sub>2</sub>, and TaS<sub>2</sub>) and selenide (TiSe<sub>2</sub>, ZrSe<sub>3</sub>, HfSe<sub>3</sub>, VSe<sub>2</sub>, NbSe<sub>2</sub>, and TaSe<sub>2</sub>)
nanocrystals. The use of appropriate chalcogen source is found to
be critical for the successful synthesis of 2-D layered TMC nanocrystals.
CS<sub>2</sub> is an efficient chalcogen precursor for metal sulfide
nanocrystals, whereas elemental Se is appropriate for metal selenide
nanocrystals. We briefly discuss the effects of reactive radical characteristics
of elemental S and Se on the formation of 2-D layered TMC nanocrystals
Enhancement of Hot Electron Flow in Plasmonic Nanodiodes by Incorporating PbS Quantum Dots
The enhancement of
hot electron generation using plasmonic nanostructures is a promising
strategy for developing photovoltaic devices. Here, we show that hot
electron flow generated in plasmonic Au/TiO<sub>2</sub> nanodiodes
by incident light can be amplified when PbS quantum dots are deposited
onto the surface of the nanodiodes. The effect is attributed to efficient
extraction of hot electrons via a three-dimensional Schottky barrier,
thus giving new pathways for hot electron transfer. We also demonstrate
a correlation between the photocurrent and Schottky barrier height
when using PbS quantum dots with varying size and ligand treatments
that allow us to control the electric properties (e.g., band gap and
Fermi level, respectively) of the PbS quantum dots. This simple method
introduces a new technique for further improving the power conversion
efficiency of thin-film photovoltaic devices
PbS Quantum Dot Solar Cells Integrated with SolāGel-Derived ZnO as an nāType Charge-Selective Layer
ZnO
thin films fabricated by a low-temperature solāgel conversion
method (L-ZnO) were employed as n-type electron-transporting layers
(ETLs) in depleted-heterojunction quantum dot solar cells (DH-QDSCs).
Thin films of PbS (ā¼200 nm) fabricated by spin-coating a colloidal
PbS QD solution layer by layer were used as the p-type photoactive
layers. The L-ZnO films functioned as efficient n-type ETLs by fully
completely depleting the PbS active layers, and they displayed performances
much higher than those of conventional ZnO nanoparticle-based ETLs.
The morphologies and chemical compositions of the L-ZnO ETLs varied
with the annealing conditions. These factors, in turn, had a marked
effect on the charge-transfer characteristics at the L-ZnO/PbS interfaces
of the DH-QDSCs. The power conversion efficiency (PCE) of the DH-QDSCs
using the optimized L-ZnO films as ETLs was 3.93%, with the fill factor
(FF) being 0.60, whereas the PCE of the cells using the ZnO nanoparticle-based
films was 1.62%, with the FF being 0.53. Thus, the solāgel-derived
L-ZnO films, which could be fabricated using a simple, low-temperature,
solution-based process, exhibited desirable performance as ETLs in
DH-QDSCs