The tens-of-percent photoluminescence (PL) quantum yields routinely obtained for colloidally prepared CdSe semiconductor nanocrystals (NCs) decrease substantially with temperature elevation. While ...such PL efficiency loss has direct consequences for applications ranging from light-emitting diodes and lasers to photovoltaics under solar concentration, the origin of this loss is currently not established, hindering synthetic efforts to design materials with robust performance. Here, for the first time, we utilize transient absorption and ultrafast PL in addition to static PL and time-correlated single photon counting, to characterize CdSe core-only and CdSe/ZnS core/shell NCs up to temperatures as high as 800 K. For multiple particle sizes, loss of PL efficiency as a function of temperature elevation is more severe and less reversible for core-only NCs than for core/shell NCs. Ultrafast measurements performed at elevated sample temperatures indicate that thermally activated trapping of individual carriers dominates the nonradiative loss of excitons. Through a combination of spectroscopic techniques, we identify the primary carrier loss process as hole trapping in particular. These findings support the notion that extrinsic trapping effects out-compete intrinsic exciton deactivation at high temperature and point to realizable improvements in thermally robust optoelectronic performance.
We report here detailed in situ studies of nucleation and growth of Au on CdSe/CdS nanorods using synchrotron SAXS technique and time-resolved spectroscopy. We examine structural and optical ...properties of CdSe/CdS/Au heterostructures formed under UV illumination. We compare the results for CdSe/CdS/Au heterostructures with the results of control experiments on CdSe/CdS nanorods exposed to gold precursor under conditions when no such heterostructures are formed (no UV illumination). Our data indicate similar photoluminescence (PL) quenching and PL decay profiles in both types of samples. Via transient absorption and PL, we show that such behavior is consistent with rapid (faster than 3 ps) hole trapping by gold–sulfur sites at the surface of semiconductor nanoparticles. This dominant process was overlooked in previous end-point studies on semiconductor/metal heterostructures.
Through mapping of the spatiotemporal strain profile in ferroelectric BiFeO3 epitaxial thin films, we report an optically initiated dynamic enhancement of the strain gradient of 10(5)-10(6) m(-1) ...that lasts up to a few ns depending on the film thickness. Correlating with transient optical absorption measurements, the enhancement of the strain gradient is attributed to a piezoelectric effect driven by a transient screening field mediated by excitons. These findings not only demonstrate a new possible way of controlling the flexoelectric effect, but also reveal the important role of exciton dynamics in photostriction and photovoltaic effects in ferroelectrics.
Botulinum neurotoxin (BoNT) presents a significant hazard under numerous realistic scenarios. The standard detection scheme for this fast-acting toxin is a lab-based mouse lethality assay that is ...sensitive and specific, but slow (∼2 days) and requires expert administration. As such, numerous efforts have aimed to decrease analysis time and reduce complexity. Here, we describe a sensitive ratiometric fluorescence resonance energy transfer scheme that utilizes highly photostable semiconductor quantum dot (QD) energy donors and chromophore conjugation to compact, single chain variable antibody fragments (scFvs) to yield a fast, fieldable sensor for BoNT with a 20–40 pM detection limit, toxin quantification, adjustable dynamic range, sensitivity in the presence of interferents, and sensing times as fast as 5 min. Through a combination of mutations, we achieve stabilized scFv denaturation temperatures of more than 60 °C, which bolsters fieldability. We also describe adaptation of the assay into a microarray format that offers persistent monitoring, reuse, and multiplexing.
The intrinsic properties of quantum dots (QDs) and the growing ability to interface them controllably with living cells has far-reaching potential applications in probing cellular processes such as ...membrane action potential. We demonstrate that an electric field typical of those found in neuronal membranes results in suppression of the QD photoluminescence (PL) and, for the first time, that QD PL is able to track the action potential profile of a firing neuron with millisecond time resolution. This effect is shown to be connected with electric-field-driven QD ionization and consequent QD PL quenching, in contradiction with conventional wisdom that suppression of the QD PL is attributable to the quantum confined Stark effect.
Three tetraalkylammonium uranyl isothiocyanates, (CH3)4N3UO2(NCS)5 (1), (C2H5)4N3UO2(NCS)5 (2), and (C3H7)4N3UO2(NCS)5 (3), have been synthesized from aqueous solution and their structures determined ...by single-crystal X-ray diffraction. All of the compounds consist of the uranyl cation equatorially coordinated to five N-bound thiocyanate ligands, UO2(NCS)5 3–, and charge-balanced by three tetraalkylammonium cations. Raman spectroscopy data have been collected on compounds 1–3, as well as on solutions of uranyl nitrate with increasing levels of sodium thiocyanate. By tracking the Raman signatures of thiocyanate, the presence of both free and bound thiocyanate is confirmed in solution. The shift in the Raman signal of the uranyl symmetric stretching mode suggests the formation of higher-order uranyl thiocyanate complexes in solution, while the solid-state Raman data support homoleptic isothiocyanate coordination about the uranyl cation. Presented here are the syntheses and crystal structures of 1–3, pertinent Raman spectra, and a discussion regarding the relationship of these isothiocyanates to previously described uranyl halide phases, UO2X4 2–.
Bright Type II Quantum Dots Tyrakowski, Christina M; Shamirian, Armen; Rowland, Clare E ...
Chemistry of materials,
11/2015, Letnik:
27, Številka:
21
Journal Article
Recenzirano
It is a paradigm that the exciton splitting characteristics of type II quantum dots (QDs) result in low quantum yield materials. However, reported herein is the synthesis of ZnSe/CdS/ZnS type II QDs ...with quantum yields as high as 61%. Most interesting is the fact that the enhancement in brightness is due to an increase in the radiative rate, rather than the suppression of surface trap-related nonradiative processes, which was found to be minimal. These QDs have been water-solubilized using two different methods and retain a substantial fraction of their brightness. The water-soluble type II QDs were conjugated to a dye to examine their potential as fluorescence resonance energy transfer (FRET) donors. A significant FRET efficiency of 61 ± 3% was measured using time-correlated single photon counting, which demonstrates the potential for these materials to be used in FRET-based chemical and biological sensing applications.
Three disulfide-containing uranyl compounds, UO2(C7H4O2S)3·H2O (1), UO2(C7H4O2S)2(C7H5O2S) (2), and UO2(C7H4O2S)4 (3) have been hydrothermally synthesized. Both in situ disulfide bond formation from ...3- and 4-mercaptobenzoic acid (C7H5O2S, MBA) to yield 3,3′- and 4,4′-dithiobisbenzoic acid (C14H8O4S2, DTBA) and direct assembly with the presynthesized dimeric ligands have been explored. While the starting materials 4-MBA and 4,4′-DTBA both yield 2 via in situ ligand synthesis and direct assembly, respectively, we observe the formation of 1 from the starting material 3-MBA via in situ ligand synthesis and of 3 from the direct assembly of the uranyl cation with 3,3′-DTBA. Concurrently with the synthesis of 1 and 2, we have observed the in situ formation of the crystalline dimeric organic species, 3,3′-DTBA, (C7H5O2S)2 (4) and 4,4′-DTBA, (C7H5O2S)2 (5). Herein we report the synthesis and crystallographic characterization of 1−5, as well as observations regarding the utility of product formation via direct assembly and in situ ligand synthesis.
Ultrafast optical pump, X-ray diffraction probe experiments were performed on CdSe nanocrystal (NC) colloidal dispersions as functions of particle size, polytype, and pump fluence. Bragg peak shifts ...related to heating and peak amplitude reduction associated with lattice disordering are observed. For smaller NCs, melting initiates upon absorption of as few as ∼15 electron–hole pair excitations per NC on average (0.89 excitations/nm3 for a 1.5 nm radius) with roughly the same excitation density inducing melting for all examined NCs. Diffraction intensity recovery kinetics, attributable to recrystallization, occur over hundreds of picoseconds with slower recoveries for larger particles. Zincblende and wurtzite NCs revert to initial structures following intense photoexcitation suggesting melting occurs primarily at the surface, as supported by simulations. Electronic structure calculations relate significant band gap narrowing with decreased crystallinity. These findings reflect the need to consider the physical stability of nanomaterials and related electronic impacts in high intensity excitation applications such as lasing and solid-state lighting.
SARS-CoV-2 infection has been shown to damage multiple organs, including the brain. Multiorgan MRI can provide further insight on the repercussions of COVID-19 on organ health but requires a balance ...between richness and quality of data acquisition and total scan duration. We adapted the UK Biobank brain MRI protocol to produce high-quality images while being suitable as part of a post-COVID-19 multiorgan MRI exam. The analysis pipeline, also adapted from UK Biobank, includes new imaging-derived phenotypes (IDPs) designed to assess the possible effects of COVID-19. A first application of the protocol and pipeline was performed in 51 COVID-19 patients post-hospital discharge and 25 controls participating in the Oxford C-MORE study. The protocol acquires high resolution T
, T
-FLAIR, diffusion weighted images, susceptibility weighted images, and arterial spin labelling data in 17 min. The automated imaging pipeline derives 1,575 IDPs, assessing brain anatomy (including olfactory bulb volume and intensity) and tissue perfusion, hyperintensities, diffusivity, and susceptibility. In the C-MORE data, IDPs related to atrophy, small vessel disease and olfactory bulbs were consistent with clinical radiology reports. Our exploratory analysis tentatively revealed some group differences between recovered COVID-19 patients and controls, across severity groups, but not across anosmia groups. Follow-up imaging in the C-MORE study is currently ongoing, and this protocol is now being used in other large-scale studies. The protocol, pipeline code and data are openly available and will further contribute to the understanding of the medium to long-term effects of COVID-19.
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