5 research outputs found
Broadband Near-Infrared to Visible Upconversion in Quantum Dot–Quantum Well Heterostructures
Upconversion is a nonlinear process
in which two, or more, long
wavelength photons are converted to a shorter wavelength photon. It
holds great promise for bioimaging, enabling spatially resolved imaging
in a scattering specimen and for photovoltaic devices as a means to
surpass the Shockley–Queisser efficiency limit. Here, we present
dual near-infrared and visible emitting PbSe/CdSe/CdS nanocrystals
able to upconvert a broad range of NIR wavelengths to visible emission
at room temperature. The synthesis is a three-step process, which
enables versatility and tunability of both the visible emission color
and the NIR absorption edge. Using this method, one can achieve a
range of desired upconverted emission peak positions with a suitable
NIR band gap
Studying Quantum Dot Blinking through the Addition of an Engineered Inorganic Hole Trap
An all-inorganic compound colloidal quantum dot incorporating a highly emissive CdSe core, which is linked by a CdS tunneling barrier to an engineered charge carrier trap composed of PbS, is designed, and its optical properties are studied in detail at the single-particle level. Study of this structure enables a deeper understanding of the link between photoinduced charging and surface trapping of charge carriers and the phenomenon of quantum dot blinking. In the presence of the hole trap, a “gray” emissive state appears, associated with charging of the core. Rapid switching is observed between the “on” and the “gray” state, although the switching dynamics in and out of the dark “off” state remain unaffected. This result completes the links in the causality chain connecting charge carrier trapping, charging of QDs, and the appearance of a “gray” emission state
Charge Transfer Dynamics in CdS and CdSe@CdS Based Hybrid Nanorods Tipped with Both PbS and Pt
The synthesis of
hybrid nanostructures that have specific properties
has become a significant topic for construction of “smart”
nanomaterials for various applications. Formation of hybrid nanostructures,
particularly those combining metals and semiconductors, often introduces
new chemical, optical, and electronic properties. Here, we show a
simple solution phase synthesis of multicomponent heterostructures
based on the growth of metal and semiconductor onto the tips of semiconductor
nanorods, leading to the formation of a hybrid semiconductor/semiconductor/metal
structure. The synthesis involves the reduction of Pt–acetylacetonate
to achieve selective growth of a Pt metal tip onto one side of the
CdS rod, followed by the thermal decomposition of Pb–bisÂ(diethyldithiocarbamate)
to grow a PbS nanocrystal onto the other tip of the nanorod. The band
alignment between the two semiconductor components as well as the
alignment with the Fermi level of the metal could support intraparticle
charge transfer, which is often sought after for photocatalysis applications.
Yet, we show, using femtosecond transient differential absorption
spectroscopy (TDA), that carrier dynamics in such a hybrid system
can be more complex than that predicted simply by bulk band alignment
considerations. This highlights the importance of the design of band
alignment and interface passivation and its role in affecting carrier
dynamics within hybrid nanostructures
A Mechanistic Study of Phase Transformation in Perovskite Nanocrystals Driven by Ligand Passivation
Active
control over the shape, composition, and crystalline habit
of nanocrystals has long been a goal. Various methods have been shown
to enable postsynthesis modification of nanoparticles, including the
use of the Kirkendall effect, galvanic replacement, and cation or
anion exchange, all taking advantage of enhanced solid-state diffusion
on the nanoscale. In all these processes, however, alteration of the
nanoparticles requires introduction of new precursor materials. Here
we show that for cesium lead halide perovskite nanoparticles, a reversible
structural and compositional change can be induced at room temperature
solely by modification of the ligand shell composition in solution.
The reversible transformation of cubic CsPbX<sub>3</sub> nanocrystals
to rhombohedral Cs<sub>4</sub>PbX<sub>6</sub> nanocrystals is achieved
by controlling the ratio of oleylamine to oleic acid capping molecules.
High-resolution transmission electron microscopy investigation of
Cs<sub>4</sub>PbX<sub>6</sub> reveals the growth habit of the rhombohedral
crystal structure is composed of a zero-dimensional layered network
of isolated PbX<sub>6</sub> octahedra separated by Cs cation planes.
The reversible transformation between the two phases involves an exfoliation
and recrystalliztion process. This scheme enables fabrication of high-purity
monodispersed Cs<sub>4</sub>PbX<sub>6</sub> nanoparticles with controlled
sizes. Also, depending on the final size of the Cs<sub>4</sub>PbX<sub>6</sub> nanoparticles as tuned by the reaction time, the back reaction
yields CsPbX<sub>3</sub> nanoplatelets with a controlled thickness.
In addition, detailed surface analysis provides insight into the impact
of the ligand composition on surface stabilization that, consecutively,
acts as the driving force in phase and shape transformations in cesium
lead halide perovskites
Infrared nanosensors of pico- to micro-newton forces
Mechanical force is an essential feature for many physical and biological processes.1-12 Remote measurement of mechanical signals with high sensitivity and spatial resolution is needed for diverse applications, including robotics,13 biophysics,14-20 energy storage,21-24 and medicine.25-27 Nanoscale luminescent force sensors excel at measuring piconewton forces,28-32 while larger sensors have proven powerful in probing micronewton forces.33,34 However, large gaps remain in the force magnitudes that can be probed remotely from subsurface or interfacial sites, and no individual, non-invasive sensor is capable of measuring over the large dynamic range needed to understand many systems.35,36 Here, we demonstrate Tm3+-doped avalanching nanoparticle37 force sensors that can be addressed remotely by deeply penetrating near-infrared (NIR) light and can detect piconewton to micronewton forces with a dynamic range spanning more than four orders of magnitude. Using atomic force microscopy coupled with single-nanoparticle optical spectroscopy, we characterize the mechanical sensitivity of the photon avalanching process and reveal its exceptional force responsiveness. By manipulating the Tm3+ concentrations and energy transfer within the nanosensors, we demonstrate different optical force-sensing modalities, including mechanobrightening and mechanochromism. The adaptability of these nanoscale optical force sensors, along with their multiscale sensing capability, enable operation in the dynamic and versatile environments present in real-world, complex structures spanning biological organisms to nanoelectromechanical systems (NEMS)