108 research outputs found
Expanding the I-II-V Phase Space: Soft Synthesis of Polytypic Ternary and Binary Zinc Antimonides
Soft chemistry methods offer the possibility of synthesizing metastable and kinetic products that are unobtainable through thermodynamically-controlled, high-temperature reactions. A recent solution-phase exploration of Li-Zn-Sb phase space revealed a previously unknown cubic half-Heusler MgAgAs-type LiZnSb polytype. Interestingly, this new cubic phase was calculated to be the most thermodynamically stable, despite prior literature reporting only two other ternary phases (the hexagonal half-Heusler LiGaGe-type LiZnSb, and the full-Heusler Li2ZnSb). This surprising discovery, coupled with the intriguing optoelectronic and transport properties of many antimony containing Zintl phases, required a thorough exploration of syn-thetic parameters. Here, we systematically study the effects that different precursor concentrations, injection order, nucleation and growth temperatures, and reaction time have on the solution-phase synthesis of these materials. By doing so, we identify conditions that selectively yield several unique ternary (c-LiZnSb vs. h*-LiZnSb), binary (ZnSb vs. Zn8Sb7), and metallic (Zn, Sb) products. Further, we find one of the ternary phases adopts a variant of the previously observed hexagonal LiZnSb struc-ture. Our results demonstrate the utility of low temperature solution phaseâsoft synthesisâmethods in accessing and mining a rich phase space. We anticipate that this work will motivate further exploration of multinary I-II-V compounds, as well as encourage similarly thorough investigations of related Zintl systems by solution phase methods
A generalized approach to photon avalanche upconversion in luminescent nanocrystals
Photon avalanching nanoparticles (ANPs) exhibit extremely nonlinear
upconverted emission valuable for sub-diffraction imaging, nanoscale sensing,
and optical computing. Avalanching has been demonstrated with Tm3+, Nd3+ or
Pr3+-doped nanocrystals, but their emission is limited to 600 and 800 nm,
restricting applications. Here, we utilize Gd3+-assisted energy migration to
tune the emission wavelengths of Tm3+-sensitized ANPs and generate highly
nonlinear emission of Eu3+, Tb3+, Ho3+, and Er3+ ions. The upconversion
intensities of these spectrally discrete ANPs scale with the nonlinearity
factor s = 10-17 under 1064 nm excitation at power densities as low as 6
kW/cm2. This strategy for imprinting avalanche behavior on remote emitters can
be extended to fluorophores adjacent to ANPs, as we demonstrate with
CdS/CdSe/CdS core/shell/shell quantum dots. ANPs with rationally designed
energy transfer networks provide the means to transform conventional linear
emitters into a highly nonlinear ones, expanding the use of photon avalanching
in biological, chemical, and photonic applications.Comment: 13 pages, 5 figure
Millisecond Kinetics of Nanocrystal Cation Exchange Using Microfluidic X-ray Absorption Spectroscopy
Giant nonlinear optical responses from photon avalanching nanoparticles
Avalanche phenomena leverage steeply nonlinear dynamics to generate
disproportionately high responses from small perturbations and are found in a
multitude of events and materials, enabling technologies including optical
phase-conjugate imaging, infrared quantum counting, and efficient upconverted
lasing. However, the photon avalanching (PA) mechanism underlying these optical
innovations has been observed only in bulk materials and aggregates, and
typically at cryogenic temperatures, limiting its utility and impact. Here, we
report the realization of PA at room temperature in single
nanostructures--small, Tm-doped upconverting nanocrystals--and demonstrate
their use in superresolution imaging at near-infrared (NIR) wavelengths within
spectral windows of maximal biological transparency. Avalanching nanoparticles
(ANPs) can be pumped by continuous-wave or pulsed lasers and exhibit all of the
defining features of PA. These hallmarks include excitation power thresholds,
long rise time at threshold, and a dominant excited-state absorption that is
>13,000x larger than ground-state absorption. Beyond the avalanching threshold,
ANP emission scales nonlinearly with the 26th power of pump intensity. This
enables the realization of photon-avalanche single-beam superresolution imaging
(PASSI), achieving sub-70 nm spatial resolution using only simple scanning
confocal microscopy and before any computational analysis. Pairing their steep
nonlinearity with existing superresolution techniques and computational
methods, ANPs allow for imaging with higher resolution and at ca. 100-fold
lower excitation intensities than is possible with other probes. The low PA
threshold and exceptional photostability of ANPs also suggest their utility in
a diverse array of applications including sub-wavelength bioimaging, IR
detection, temperature and pressure transduction, neuromorphic computing, and
quantum optics.Comment: 14 pages, 4 figure
Precursor reaction kinetics control compositional grading and size of CdSe1-xSx nanocrystal heterostructures
We report a method to control the composition and microstructure of CdSe1-xSx nanocrystals by the simultaneous injection of sulfide and selenide precursors into a solution of cadmium oleate and oleic acid at 240 degrees C. Pairs of substituted thio- and selenoureas were selected from a library of compounds with conversion reaction reactivity exponents (k(E)) spanning 1.3 x 10(-5) s(-1) to 2.0 x 10(-1) s(-1). Depending on the relative reactivity (k(Se)/k(S)), core/shell and alloyed architectures were obtained. Growth of a thick outer CdS shell using a syringe pump method provides gram quantities of brightly photoluminescent quantum dots (PLQY = 67 to 90%) in a single reaction vessel. Kinetics simulations predict that relative precursor reactivity ratios of less than 10 result in alloyed compositions, while larger reactivity differences lead to abrupt interfaces. CdSe1-xSx alloys (k(Se)/k(S) = 2.4) display two longitudinal optical phonon modes with composition dependent frequencies characteristic of the alloy microstructure. When one precursor is more reactive than the other, its conversion reactivity and mole fraction control the number of nuclei, the final nanocrystal size at full conversion, and the elemental composition. The utility of controlled reactivity for adjusting alloy microstructure is discussed
Indefinite and Bidirectional Near Infrared Nanocrystal Photoswitching
Materials whose luminescence can be switched by optical stimulation drive
technologies ranging from superresolution imaging1-4, nanophotonics5, and
optical data storage6-8, to targeted pharmacology, optogenetics, and chemical
reactivity9. These photoswitchable probes, including organic fluorophores and
proteins, are prone to photodegradation, and often require phototoxic doses of
ultraviolet (UV) or visible light. Colloidal inorganic nanoparticles have
significant stability advantages over existing photoswitchable materials, but
the ability to switch emission bidirectionally, particularly with NIR light,
has not been reported with nanoparticles. Here, we present 2-way, near-infrared
(NIR) photoswitching of avalanching nanoparticles (ANPs), showing full optical
control of upconverted emission using phototriggers in the NIR-I and NIR-II
spectral regions useful for subsurface imaging. Employing single-step
photodarkening10-13 and photobrightening12,14-18, we demonstrate indefinite
photoswitching of individual nanoparticles (>1000 cycles over 7 h) in ambient
or aqueous conditions without measurable photodegradation. Critical steps of
the photoswitching mechanism are elucidated by modeling and by measuring the
photon avalanche properties of single ANPs in both bright and dark states.
Unlimited, reversible photoswitching of ANPs enables indefinitely rewritable 2D
and 3D multi-level optical patterning of ANPs, as well as optical nanoscopy
with sub-{\AA} localization superresolution that allows us to distinguish
individual ANPs within tightly packed clusters.Comment: 15 pages, 5 figure
Enhancing FRET biosensing beyond 10 nm with photon avalanche nanoparticles.
Förster Resonance Energy Transfer (FRET) between donor (D) and acceptor (A) molecules is a phenomenon commonly exploited to study or visualize biological interactions at the molecular level. However, commonly used organic D and A molecules often suffer from photobleaching and spectral bleed-through, and their spectral properties hinder quantitative analysis. Lanthanide-doped upconverting nanoparticles (UCNPs) as alternative D species offer significant improvements in terms of photostability, spectral purity and background-free luminescence detection, but they bring new challenges related to multiple donor ions existing in a single large size UCNP and the need for nanoparticle biofunctionalization. Considering the relatively short Förster distance (typically below 5-7 nm), it becomes a non-trivial task to assure sufficiently strong D-A interaction, which translates directly to the sensitivity of such bio-sensors. In this work we propose a solution to these issues, which employs the photon avalanche (PA) phenomenon in lanthanide-doped materials. Using theoretical modelling, we predict that these PA systems would be highly susceptible to the presence of A and that the estimated sensitivity range extends to distances 2 to 4 times longer (i.e. 10-25 nm) than those typically found in conventional FRET systems. This promises high sensitivity, low background and spectral or temporal biosensing, and provides the basis for a radically novel approach to combine luminescence imaging and self-normalized bio-molecular interaction sensing
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