Nanocrystal
(NC) solids are commonly prepared from nonpolar organic
NC suspensions. In many cases, the capping on the NC surface is preserved
and forms a barrier between the NCs. More recently, superstructures
with crystalline connections between the NCs, implying the removal
of the capping, have been reported, too. Here, we present large-scale
uniform superstructures of attached PbSe NCs with a silicene-type
honeycomb geometry, resulting from solvent evaporation under nearly
reversible conditions. We also prepared multilayered silicene honeycomb
structures by using larger amounts of PbSe NCs. We show that the two-dimensional
silicene superstructures can be seen as a crystallographic slice from
a 3-D simple cubic structure. We describe the disorder in the silicene
lattices in terms of the nanocrystals position and their atomic alignment.
The silicene honeycomb sheets are large enough to be used in transistors
and optoelectronic devices.
Self-assembled nanocrystal
solids show promise as a versatile platform
for novel optoelectronic materials. Superlattices composed of a single
layer of lead–chalcogenide and cadmium–chalcogenide
nanocrystals with epitaxial connections between the nanocrystals,
present outstanding questions to the community regarding their predicted
band structure and electronic transport properties. However, the as-prepared
materials are intrinsic semiconductors; to occupy the bands in a controlled
way, chemical doping or external gating is required. Here, we show
that square superlattices of PbSe nanocrystals can be incorporated
as a nanocrystal monolayer in a transistor setup with an electrolyte
gate. The electron (and hole) density can be controlled by the gate
potential, up to 8 electrons per nanocrystal site. The electron mobility
at room temperature is 18 cm2/(V s). Our work forms a first
step in the investigation of the band structure and electronic transport
properties of two-dimensional nanocrystal superlattices with controlled
geometry, chemical composition, and carrier density.
Lead halide perovskite
nanocrystals have drawn attention as active
light-absorbing or -emitting materials for opto-electronic applications
due to their facile synthesis, intrinsic defect tolerance, and color-pure
emission ranging over the entire visible spectrum. To optimize their
application in, e.g., solar cells and light-emitting diodes, it is
desirable to gain control over electronic doping of these materials.
However, predominantly due to the intrinsic instability of perovskites,
successful electronic doping has remained elusive. Using spectro-electrochemistry
and electrochemical transistor measurements, we demonstrate here that
CsPbBr
3
nanocrystals can be successfully and reversibly
p-doped via electrochemical hole injection. From an applied potential
of ∼0.9 V vs NHE, the emission quenches, the band edge absorbance
bleaches, and the electronic conductivity quickly increases, demonstrating
the successful injection of holes into the valence band of the CsPbBr
3
nanocrystals.
Low-dimensional semiconductors have found numerous applications in optoelectronics. However, a quantitative comparison of the absorption strength of lowdimensional versus bulk semiconductors has remained elusive. Here, we report generality in the band-edge light absorptance of semiconductors, independent of their dimensions. First, we provide atomistic tight-binding calculations that show that the absorptance of semiconductor quantum wells equals mπα (m = 1 or 2 with α as the fine-structure constant), in agreement with reported experimental results. Then, we show experimentally that a monolayer (superlattice) of quantum dots has similar absorptance, suggesting an absorptance quantum of mπα per (confined) exciton diameter. Extending this idea to bulk semiconductors, we experimentally demonstrate that an absorptance quantum equal to mπα per exciton Bohr diameter explains their widely varying absorption coefficients. We thus provided compelling evidence that the absorptance quantum πα per exciton diameter rules the band-edge absorption of all direct semiconductors, regardless of their dimension.
It
has been shown recently that atomically coherent superstructures of
a nanocrystal monolayer in thickness can be prepared by self-assembly
of monodisperse PbSe nanocrystals, followed by oriented attachment.
Superstructures with a honeycomb nanogeometry are of special interest,
as theory has shown that they are regular 2-D semiconductors, but
with the highest valence and lowest conduction bands being Dirac-type,
that is, with a linear energy-momentum relation around the K-points
in the zone. Experimental validation will require cryogenic measurements
on single sheets of these nanocrystal monolayer superstructures. Here,
we show that we can incorporate these fragile superstructures into
a transistor device with electrolyte gating, control the electron
density, and measure the electron transport characteristics at room
temperature. The electron mobility is 1.5 ± 0.5 cm2 V–1 s–1, similar to the mobility
observed with terahertz spectroscopy on freestanding superstructures.
The terahertz spectroscopic data point to pronounced carrier scattering
on crystallographic imperfections in the superstructure, explaining
the limited mobility.
Quantum dots (QDs)
are considered for devices like light-emitting
diodes (LEDs) and photodetectors as a result of their tunable optoelectronic
properties. To utilize the full potential of QDs for optoelectronic
applications, control over the charge carrier density is vital. However,
controlled electronic doping of these materials has remained a long-standing
challenge, thus slowing their integration into optoelectronic devices.
Electrochemical doping offers a way to precisely and controllably
tune the charge carrier concentration as a function of applied potential
and thus the doping levels in QDs. However, the injected charges are
typically not stable after disconnecting the external voltage source
because of electrochemical side reactions with impurities or with
the surfaces of the QDs. Here, we use photopolymerization to covalently
bind polymerizable electrolyte ions to polymerizable solvent molecules
after electrochemical charge injection. We discuss the importance
of using polymerizable dopant ions as compared to nonpolymerizable
conventional electrolyte ions such as LiClO
4
when used
in electrochemical doping. The results show that the stability of
charge carriers in QD films can be enhanced by many orders of magnitude,
from minutes to several weeks, after photochemical ion fixation. We
anticipate that this novel way of stable doping of QDs will pave the
way for new opportunities and potential uses in future QD electronic
devices.
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