24 research outputs found
Size-Dependent Lattice Structure and Confinement Properties in CsPbIā Perovskite Nanocrystals: Negative Surface Energy for Stabilization
CsPbIā nanocrystals with narrow size distributions were prepared to study the size-dependent properties. The nanocrystals adopt the perovskite (over the nonperovskite orthorhombic) structure with improved stability over thin-film materials. Among the perovskite phases (cubic Ī±, tetragonal Ī², and orthorhombic Ī³), the samples are characterized by the Ī³ phase, rather than Ī±, but may have a size-dependent average tilting between adjacent octahedra. Size-dependent lattice constants systematically vary 3% across the size range, with unit cell volume increasing linearly with the inverse of size to 2.1% for the smallest size. We estimate the surface energy to be from ā3.0 to ā5.1 eV nmā»Ā² for ligated CsPbIā nanocrystals. Moreover, the size-dependent bandgap is best described using a nonparabolic intermediate confinement model. We experimentally determine the bulk bandgap, effective mass, and exciton binding energy, concluding with variations from the bulk Ī±-phase values. This provides a robust route to understanding Ī³-phase properties of CsPbIā
Embedding PbS Quantum Dots (QDs) in Pb-Halide Perovskite Matrices: QD Surface Chemistry and Antisolvent Effects on QD Dispersion and Confinement Properties
Hybrid materials of metal chalcogenide colloidal quantum dots (QDs) embedded in metal halide perovskites (MHPs) have led to composites with synergistic properties. Here, we investigate how QD size, surface chemistry, and MHP film formation methods affect the resulting optoelectronic properties of QD/MHP ādot-in-matrixā systems. We monitor the QD absorption and photoluminescence throughout synthesis, ligand exchange, and transfer into the MHP ink, and we characterize the final QD/MHP films via electron microscopy and transient absorption. In addition, we are the first to globally map how PbS QDs are distributed on the micrometer scale within these dot-in-matrix systems, using three-dimensional (3D) tomography time-of-flight secondary ion mass spectrometry. The surface chemistry imparted during synthesis directly affects the optical properties of the dot-in-matrix composites. Pb-halide passivation leads to QD/MHP dot-in-matrix samples with optical properties that are well-described by a theoretical model, based on a Type I finite-barrier heterostructure between the PbS QD and the MHP matrix. Samples without Pb-halide passivation show complicated size-dependent behavior, indicating a transition from a Type I heterostructure between the PbS QD wells and MHP barriers for small-sized QDs to PbS QDs that are electronically decoupled from the MHP matrix for larger QDs. Furthermore, the choice in perovskite antisolvent crystallization method leads to a difference in the spatial QD distribution within the perovskite matrix, differences in carrier lifetime, and photoluminescence shifts of up to 180 meV for PbS in methylammonium lead iodide. This work establishes an understanding of such emerging synergistic systems relevant for technologies such as photovoltaics, infrared emitters and detectors, and other unexplored technological applications
Spectrally-Resolved Dielectric Functions of Solution-Cast Quantum Dot Thin Films
Quantum confinement is the divergence,
at small crystallite size,
of the electronic structure of semiconductor nanocrystals, or quantum
dots, from the properties of larger crystals of the same materials.
Although the extinction properties of quantum dots in the dispersed
state have been extensively studied, many applications for quantum
dots require the formation of a solid material which nonetheless retains
a size-dependent electronic structure. The complex index of refraction
(or complex dielectric function), including the extinction coefficient,
is critical information for interpretation of optoelectronic measurements
and use of quantum dot solids in optoelectronic devices. Here, spectroscopic
ellipsometry is used to provide an all-optical method to determine
the thickness, complex index, and extinction coefficient of thin films
made of quantum-confined materials through the visible and near-infrared
spectral ranges. The characteristic, size-dependent spectral features
in the absorption of monodisperse quantum dots are readily translated
into spectral variations of the index of refraction. The complex indices
of refraction of CdSe and PbS quantum dot solids depend strongly on
quantum dot size and the processing conditions of the thin film, including
ligand exchange and annealing. The dielectric functions of quantum
dot solids are dominated by the fill fraction of quantum dots, with
only secondary influence from interparticle interaction
Increased Carrier Mobility and Lifetime in CdSe Quantum Dot Thin Films through Surface Trap Passivation and Doping
Passivating
surface defects and controlling the carrier concentration
and mobility in quantum dot (QD) thin films is prerequisite to designing
electronic and optoelectronic devices. We investigate the effect of
introducing indium in CdSe QD thin films on the dark mobility and
the photogenerated carrier mobility and lifetime using field-effect
transistor (FET) and time-resolved microwave conductivity (TRMC) measurements.
We evaporate indium films ranging from 1 to 11 nm in thickness on
top of approximately 40 nm thick thiocyanate-capped CdSe QD thin films
and anneal the QD films at 300 Ā°C to densify and drive diffusion
of indium through the films. As the amount of indium increases, the
FET and TRMC mobilities and the TRMC lifetime increase. The increase
in mobility and lifetime is consistent with increased indium passivating
midgap and band-tail trap states and doping the films, shifting the
Fermi energy closer to and into the conduction band
Increased Carrier Mobility and Lifetime in CdSe Quantum Dot Thin Films through Surface Trap Passivation and Doping
Passivating
surface defects and controlling the carrier concentration
and mobility in quantum dot (QD) thin films is prerequisite to designing
electronic and optoelectronic devices. We investigate the effect of
introducing indium in CdSe QD thin films on the dark mobility and
the photogenerated carrier mobility and lifetime using field-effect
transistor (FET) and time-resolved microwave conductivity (TRMC) measurements.
We evaporate indium films ranging from 1 to 11 nm in thickness on
top of approximately 40 nm thick thiocyanate-capped CdSe QD thin films
and anneal the QD films at 300 Ā°C to densify and drive diffusion
of indium through the films. As the amount of indium increases, the
FET and TRMC mobilities and the TRMC lifetime increase. The increase
in mobility and lifetime is consistent with increased indium passivating
midgap and band-tail trap states and doping the films, shifting the
Fermi energy closer to and into the conduction band
Xāray Mapping of Nanoparticle Superlattice Thin Films
We combine grazing-incidence and transmission small-angle X-ray diffraction with electron microscopy studies to characterize the structure of nanoparticle films with long-range order. Transmission diffraction is used to collect in-plane diffraction data from single grains and locally aligned nanoparticle superlattice films. Systematic mapping of samples can be achieved by translating the sample in front of the X-ray beam with a spot size selected to be on the order of superlattice grain features. This allows a statistical determination of superlattice grain size and size distribution over much larger areas than typically accessible with electron microscopy. Transmission X-ray measurements enables spatial mapping of the grain size, orientation, uniformity, strain, or crystal projections and polymorphs. We expand this methodology to binary nanoparticle superlattice and nanorod superlattice films. This study provides a framework for characterization of nanoparticle superlattices over large areas which complements or expands microstructure information from real-space imaging
Coherent acoustic phonons in colloidal semiconductor nanocrystal superlattices
The
phonon properties of films fabricated from colloidal semiconductor
nanocrystals play a major role in thermal conductance and electron
scattering, which govern the principles for building colloidal-based
electronics and optics including thermoelectric devices with a high <i>ZT</i> factor. The key point in understanding the phonon properties
is to obtain the strength of the elastic bonds formed by organic ligands
connecting the individual nanocrystallites. In the case of very weak
bonding, the ligands become the bottleneck for phonon transport between
infinitively rigid nanocrystals. In the opposite case of strong bonding,
the colloids cannot be considered as infinitively rigid beads and
the distortion of the superlattice caused by phonons includes the
distortion of the colloids themselves. We use the picosecond acoustics
technique to study the acoustic coherent phonons in superlattices
of nanometer crystalline CdSe colloids. We observe the quantization
of phonons with frequencies up to 30 GHz. The frequencies of quantized
phonons depend on the thickness of the colloidal films and possess
linear phonon dispersion. The measured speed of sound and corresponding
wave modulus in the colloidal films point on the strong elastic coupling
provided by organic ligands between colloidal nanocrystals